JP2004212501A - Light scanning apparatus and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce power variation among a plurality of beams on a scanned surface and suppress deterioration of quality of an output image, in a light scanning apparatus and an image forming apparatus provided with liquid crystal elements for controlling the positions of the light beams on the scanned surface by deflecting the light paths of the light beams from light sources. <P>SOLUTION: This is the light scanning apparatus 20 provided with the liquid crystal elements 40a, 40b for controlling the positions of the light beams on the scanned surface by deflecting the light paths 21a, 21b of the light beams from the light sources 41, 42, and transmittance variation T(%) of the liquid crystal elements and the light path deflection angle θ(min) associated with the light path deflections by the liquid crystal elements 40a, 40b satisfy the relation of T/θ≤2.0(%/min). <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

である。上記条件（ｆｃｏｌ＝１５（ｍｍ）、ｍＺ＝９．５）程度の光学系の場合、被走査面での光ビーム位置の変化、つまり経時変動、環境変動、機械設置時の変動などは、経験的に数１０（μｍ）程度であるため、この光ビーム位置変化を補正するための光路偏向角は、高々θ＝２．０（分）となり問題はない。
【００２３】
この光路偏向角の調整範囲内での液晶素子の透過率変動が、光ビーム位置精度と光ビーム強度を同時に良好に維持するためには非常に重要なパラメータとなる。すなわち、液晶素子による光路偏向に伴うこの液晶素子の透過率変動Ｔ（％）と、この液晶素子による光路偏向角θ（分）が、Ｔ／θ＝４（％）／２．０（分）＝２．０（％／分）以下、望ましくは１．０（％／分）以下とすることで、要求される光ビーム位置調整範囲内での光ビーム強度変化を有効に抑制することが可能となる。
【００２４】
液晶素子での透過率が変動することによって、被走査面での光ビーム強度が変動する原因として、たとえば、以下の原因がある。
（１）マティック型液晶素子においては、液晶に印加する駆動電圧により常光線と異常光線の屈折率の異方性Δｎ（＝ｎｅ−ｎｏ）を変化させることで液晶層内に屈折率勾配を形成し、ビームを偏向している。そのため、ビーム偏向角、すなわち屈折率の異方性に従い、図３に示すように透過率が周期的に変化する。なおΔｎを変化させるには、一対のガラス基板、透明電極、配向膜等に挟まれた液晶層に、上記透明電極から電圧を印加することが一般に行われている。このようにすれば、印加電圧に比例した屈折率勾配を発生させることが可能である。
（２）液晶素子の電極パターンに起因する回折を利用してビーム偏向する構成の場合には、回折格子のピッチに従い回折効率が変化することで、透過率が変動する。
【００２５】
図３は、ビーム偏向（被走査面での光ビーム位置調整）に伴う液晶素子の透過率変動の例を示す線図であり、横軸は液晶の屈折率の異方性Δｎ（＝ｎe−ｎo）の関数ｕ＝２×Δｎ×ｄ×λで、液晶素子による光ビームの偏向角φに対応するパラメータである。ここで、ｄは液晶層の厚さ、λは光ビームの波長である。液晶素子を構成する設計定数を適切に設定し、前記条件（ｆｃｏｌ＝１５（ｍｍ）、ｍＺ＝９．５）程度の光学系の場合、液晶の透過率ηの変動幅が４％（望ましくは２％）となる領域を使用範囲とすればよい。すなわち、図３下図において、透過率が９８％〜１００％（幅：２％）となる領域Ａが液晶素子の使用範囲である。ただし、図３上図の領域Ｂでは、波面収差（球面収差）が大きくなり、光学性能が劣化するため、光路偏向素子としての使用は不可となる。ここで、液晶の透過率ηは、
η＝１−［ｓｉｎ２｛（π／２）×（１＋ｕ２）１／２｝］／（１＋ｕ２）
で表される。
【００２６】
なお、被走査面での光ビーム位置を副走査方向に、Δｚ変位するには、
Δｚ＝ｆｃｏｌ×ｍＺ×ｔａｎφＺ
より
φＺ＝ｔａｎ−１（Δｚ／ｆｃｏｌ×ｍＺ）
ｆｃｏｌ：カップリングレンズの焦点距離
ｍＹ：全系の主走査倍率
ｍＺ：全系の副走査倍率
φＹ：ビーム偏向角の主走査方向成分
φＺ：ビーム偏向角の副走査方向成分
ＦＹ：偏向器以降の光学系の主走査焦点距離
となるように、
ビーム偏向角の副走査方向成分：φＺ
を発生させればよい。
また、ビームスポット位置を主走査方向にΔｙ変位するには、
φＹ＝ｔａｎ−１（Δｙ／ｆｃｏｌ×ｍＹ）＝ｔａｎ−１（Δｙ／ＦＹ）
となるように、
ビーム偏向角の主走査方向成分：φＹ
を発生させればよい。
【００２７】
図４は、ビーム偏向に伴う液晶素子の透過率変動の別の例を示す線図である。
図４に示すような透過率変動の場合には、透過率変動が２％となる領域、つまり、透過率が９１％〜９３％（幅：２％）となる領域Ａ１〜Ａｎが液晶素子の使用範囲である。各領域Ａ１〜Ａｎは離散的になるため、使用可能な光路偏向角も離散的になる。このような場合には、設計定数を適切に設定して振動の周期を短くすることで、所望の偏向角を高分解能にて得ることができる。ただし、ビーム偏向角、すなわち図４の横軸が大きくなるに従い液晶での波面収差が大きくなるため、たとえば、図４の領域Ａｎ＋１より右側の領域は、光ビーム位置調整手段としては使用不可能な領域である。
【００２８】
なお、本発明にかかる光走査装置においては、液晶の駆動（光路偏向による光ビーム位置の調整）に伴い発生する周期的な透過率変動の周期（サイクル）が、光路偏向範囲、つまり光ビーム位置調整の範囲において１０以上現れるようにしてもよい。たとえば、前記条件（ｆｃｏｌ＝１５（ｍｍ）、ｍＺ＝９．５）程度の光学系において被走査面での光ビーム位置を９５（μｍ）移動させたい場合、つまり調整範囲が９５μｍの場合には、上記透過率変動の周期を１０以上に設定することで、光ビーム位置を最低でも９．５（μｍ）の分解能にて調整することが可能となる。
【００２９】
また、各周期における極大値がほぼ一定になるようにすれば、透過率を高い側に維持することができ、エネルギ損失の増加を回避することが可能となる。
さらに、図５に示すように、Δｎ（横軸）の増加に伴い透過率（縦軸）が周期的に振動しながら減少する傾向を示す場合には、必要なビーム偏向角の範囲にて、透過率変動が４％（望ましくは２％）となる領域、つまり、図５の例では透過率が９４％〜９６％（幅：２％）となる領域Ａが、液晶素子の使用範囲となる。
【００３０】
図３の上図に示すように、一般に液晶層に印加する電圧の増加、すなわちΔｎの増加に伴い光路偏向角も線形的に増加するが、印加電圧がある値以上になるとこの線形性が失われ、また、液晶層の波面収差が大きくなるなどの不具合が発生する（図３上図の領域Ｂ参照）。
また、図３の上図の線形領域より図の左側の領域では非線形であるため、光路偏向素子としての使用は煩雑となる。この点にも留意し、「透過率変動が２％以下となる領域」と「光路偏向角が線形性を示す領域」が広くなるように設計する必要がある。
【００３１】
以上説明した実施の形態によれば、液晶素子の光路偏向角範囲（光ビーム位置調整範囲）での透過率変動を抑制することにより、被走査面、つまり同一像高での光ビーム強度の偏差及び／又は複数ビーム間の偏差を低減することができる。
したがって、書き出し光ビームが切り替わった場合に発生する各色間のバランスのずれに起因するカラー画像品質の劣化を有効に抑制することが可能となる。
【００３２】
なお、本発明にかかる光走査装置においては、光ビーム強度検出手段により、被走査面または被走査面と等価な面にて光ビーム強度を検出し、光ビーム強度が所定値を超えた場合に液晶素子を駆動、つまり、光路偏向をし、光ビーム強度が所定値以内に入るようにしてもよい。図４に示すような透過率特性を示す場合には、光ビーム位置の調整値は離散的になる恐れがあるが、光ビーム位置調整の分解能に応じて透過率振動の周期を適宜設計すればよい。
【００３３】
また、被走査面での光ビームの強度を補正する光ビーム強度補正手段により、被走査面における光ビーム強度及び／又は複数ビーム間の光ビーム強度の偏差を更に改善する（たとえば、１％以下）ようにしてもよい。
図３の上図に示すように、一般に「液晶素子での光路偏向角」と「被走査面での光ビーム強度の変動量」の関係は、理論的（設計的）に決定される。そこで、ビーム偏向量から光パワー補正量を算出するテーブルを用意しておき、そのテーブル（図示しない）に基づき、「光ビーム強度補正手段」を駆動／制御するとよい。この場合、「光ビーム強度検出手段」を備える必要はない。
【００３４】
光ビーム強度補正手段の一例として、たとえば、光源（半導体レーザ）の発光出力を変動することができる。あるいは、光ビームの光路内にフィルタ、たとえば、光ビームが有する偏光特性を利用したフィルタや単純に光エネルギを内部で吸収したり表面で反射したりするフィルタ等を配置しても構わない。
【００３５】
しかしながら、上記理論値（設計値）は、実機においては、部品ばらつき／組み付けばらつき、又は経時的な劣化、環境（温度、湿度）変化などにより変動する恐れがある。そこで、図１に示すように、被走査面（又は光学的に等価な位置）に光ビーム強度検出手段１９を配備し、光ビーム強度検出手段１９の検出結果に基づき光ビーム強度補正手段を駆動し、または制御するとよい。
【００３６】
次に、本発明にかかる画像形成装置について説明する。
図２は、本発明にかかる画像形成装置の実施の形態を示す光学配置図である。
画像形成装置２００は、ブラック（Ｋ），シアン（Ｃ），マゼンタ（Ｍ），イエロー（Ｙ）の各色に対応した光走査装置２０Ｋ，２０，Ｃ，２０Ｍ，２０Ｙと感光体ドラム１６Ｋ，１６Ｃ，１６Ｍ，１６Ｙ、これらの感光体ドラム表面上に形成された静電潜像をトナーで顕像化する現像手段、顕像化されたトナー像を記録紙に転写する転写手段などの、電子写真プロセスを実行する手段を有してなり、複数ビームを同時に走査することによりカラー画像の出力速度の高速化／高密度化に有利なタンデム型の画像形成装置である。各感光体ドラムの表面が、前記光走査装置の被走査面となっている。このような感光手段を４つ有するカラー画像形成装置では、感光手段が１つ、つまり４色に対応して４回の書込が必要な画像形成装置と比較して、４倍の出力画像を得ることが可能となる。なお、図中、現像手段や転写手段などは図示を省略している。
【００３７】
各感光体ドラムの周囲には、図示しない帯電器、現像器、転写器など、電子写真プロセスにしたがうプロセス部材が順に配置されている。光走査装置は、電子写真プロセスの露光プロセスを実行するもので、帯電器で均一に帯電された感光体ドラムの表面を走査して静電潜像を形成する。
【００３８】
このような複数の感光体を有するタンデム型画像形成装置において、たとえば複数色モード選択時であれば、感光体ドラム１６Ｋ，１６Ｃ，１６Ｍ，１６Ｙに対して、対応する色の画像信号に応じて図示しない露光ユニットの露光により、感光体ドラム１６Ｋ，１６Ｃ，１６Ｍ，１６Ｙ上に静電潜像が形成される。これらの静電潜像は、各々の対応する色トナーで現像されてトナー像となり、転写ベルト上に静電的に吸着されて、搬送される転写紙上に順次転写されることにより重ね合わせられる。そして、定着器によりカラー画像として定着され、排紙される。なお、転写ベルト、転写紙、定着器は、図示を省略している。
また、単色モード選択時であれば、ある色（Ｋ，Ｃ，Ｍ，Ｙのいずれか）に対応する感光体ドラムおよびプロセス部材のみが動作し、他の色の感光体ドラム及びプロセス部材は非動作状態となる。
【００３９】
画像形成装置２００において、すべての光走査装置２０Ｋ、２０Ｃ、２０Ｍ、２０Ｙから出射されるビームの本数が各々１本の場合には、フルカラー（４色）画像を得ることができる。一方、４つの光走査装置の少なくとも１つ（たとえばブラックに対応する光走査装置２０Ｋ）を本発明にかかる複数（Ｎ）ビームの光走査装置とし、この光走査装置のみで光走査を行う場合、フルカラー画像時と比較してＮ倍の高密度化が可能となる。
また、記録媒体の搬送速度（及びプロセス速度）を４倍にすれば、画像出力枚数を４倍に増加することが可能となる。さらに、フルカラー画像時においても、文字画像についてはブラックにて書き込むことが多く高解像度も要求されることが多いため、上記の複数（Ｎ）ビーム光走査装置２０Ｋ（ブラック）に付加して、他の光走査装置（２０Ｃ，２０Ｍ，２０Ｙ；１ビーム）も同時に書き込むことにより、文字／写真／線画イメージ等が混在した画像においてもより高品位な出力画像を得ることが可能となる。
【００４０】
画像形成装置２００が備える各色に対応した光走査装置に本発明にかかる光走査装置を適用することで、１の感光体ドラム表面、すなわち被走査面上を走査する複数ビーム間の相対位置、つまりビームピッチを所望の値に調整することが可能となり、高品質な出力画像を得ることができる。
【００４１】
図７は、本発明にかかる画像形成装置の別の実施の形態を示す光学配置図である。この画像形成装置は、複数の光源からなる光源ユニットから出射した複数の光束が複数の被走査面上を同時に走査するタンデム型の画像形成装置である。偏向手段としてポリゴンミラー、走査光学系の構成などは公知のものと変わりないから説明は省略する。感光体ドラム１６Ｋ，１６Ｃ，１６Ｍ，１６Ｙに至る光ビームの光路内には、各感光体ドラム上の光ビーム位置を調整する液晶素子４０が配設されている。液晶素子４０は、１つの素子内に複数の有効エリアを有するものでも良いし、あるいは、光ビーム毎に１つの有効エリアを有する独立した素子としても構わない。
【００４２】
この画像形成装置に本発明にかかる光走査装置を適用すれば、液晶素子４０により複数の感光体ドラム間の相対的な光ビーム位置の補正が可能である。すなわち、たとえば、転写ベルト３１上に形成されたトナー像３２を色ずれ検知用センサ３３にて検出し、その検出結果、つまりステーション間の色ずれの程度に基づき液晶素子４０を駆動することにより、各ステーションの副走査方向（あるいは主走査方向でも構わない）の書込開始タイミング（すなわち書込開始位置）を補正することができる。したがって、転写ベルト３１上でのトナー像３２の色ずれを低減でき、高品質な出力画像を得ることができる。
【００４３】
ここで、複写機、プリンタ等にて数十枚以上の連続プリントを行った場合、機内の温度上昇等の影響により、ステーション間の色ずれが発生する恐れがある。このような色ずれを補正するために上記の液晶素子を利用した場合、補正量に従い透過率変動、すなわち感光体ドラム上での光ビーム強度の変動が発生する恐れがあった。しかしながら、本発明にかかる光走査装置を用いれば、この光ビーム強度の変動は抑制できるため、連続プリント時でも出力画像品質の低下を抑制することが可能である。
【００４４】
なお、連続プリント時の変動だけではなく、機械移動／設置時の変動や経時的な変動等の補正に使用することも可能である。
また、液晶素子は、全ての光ビームの光路に配設する必要はない。ある基準色（たとえばブラック）に対して位置合わせするために、他の色（たとえば、シアン、マゼンタ、イエロー）の光路内にのみ液晶素子を配設すればよい。
【００４５】
図６は、本発明にかかる画像形成装置のさらに別の実施の形態を示す偏向回転面に平行な面内に展開した光学配置図である。この画像形成装置は、複数の光走査装置２０を主走査方向に並列して配備し、有効書込幅を分割して被走査面１６上を走査するようにしたものである。
この画像形成装置に本発明にかかる光走査装置を適用すれば、被走査面１６上の分割走査の繋ぎ部における光ビーム位置（初期調整時、環境／経時変動など）を補正することができる。有効書込幅を大きくすることができれば、光走査装置を構成する光学素子や偏向器の小型化が可能であるため、機械公差や温度変動によるビームウエスト位置の変動が小さくなり、波面収差を低減することができ、その結果、高品質の出力画像を得ることができる。
【００４６】
【発明の効果】
本発明によれば、光源からの光ビームの光路を偏向して被走査面上の光ビーム位置を制御する液晶素子の光路偏向角範囲での透過率変動を抑制したので、被走査面での複数ビーム間のパワー偏差を低減することができ、出力画像の品質の劣化を抑制することができる。
【図面の簡単な説明】
【図１】本発明にかかる光走査装置の実施の形態を示す偏向反射面に平行な面内に展開した光学配置図である。
【図２】本発明かかる画像形成装置の実施の形態を示す光学配置図である。
【図３】ビーム偏向に伴う液晶素子の透過率変動の例を示す線図である。
【図４】ビーム偏向に伴う液晶素子の透過率変動の別の例を示す線図である。
【図５】ビーム偏向に伴う液晶素子の透過率変動のさらに別の例を示す線図である。
【図６】本発明かかる画像形成装置の別の実施の形態を示す光学配置図である。
【図７】本発明かかる画像形成装置のさらに別の実施の形態を示す光学配置図である。
【符号の説明】
１３ シリンドリカルレンズ
１４ ポリゴンミラー
１５ 走査光学系
１６ 感光体ドラム
１９ 光ビーム強度検出手段
２０ 光走査装置
４０ａ，４０ｂ 液晶素子
４１，４２ 光源部[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical scanning device used for a laser beam printer (LBP), a digital copying machine, a laser facsimile, and the like, and an image forming apparatus provided with the optical scanning device.
[0002]
[Prior art]
The tandem-type full-color image forming apparatus has four photosensitive drums arranged in line along the conveying surface of the intermediate transfer belt corresponding to each color of cyan (C), magenta (M), yellow (Y), and black (K). A beam is scanned by a beam scanning device provided for each photosensitive drum to form an electrostatic latent image on the peripheral surface of the photosensitive drum, and the electrostatic latent image is formed with a toner of a corresponding color. A multicolor image is formed by visualizing the image and sequentially transferring the image on a sheet conveyed by an intermediate transfer belt.
As the scanning means in the above beam scanning device, a rotation driving polygon mirror is usually used at a predetermined rotation speed by a motor. The beam scanning device has a line period signal generating means, which detects a laser beam from the scanning means at a predetermined position and generates a line synchronizing signal. The laser beam is modulated by the image signal in synchronization with the line synchronization signal, and an image is written for each line. The intermediate transfer reference signal generating means detects a mark on the intermediate transfer body at a predetermined position, generates an intermediate transfer reference signal, and performs an image forming operation of each color to form a toner image of each color on the photosensitive drum. This is performed in synchronization with the reference signal.
[0003]
In such a color image forming apparatus, since the intermediate transfer reference signal and the line synchronization signal are asynchronous, the phase of the intermediate transfer reference signal and the line synchronization signal may be greatly shifted as the number of laser beams increases, The shift in the image writing start position in the sub-scanning direction becomes large, causing a color shift, that is, a shift in the position of the toner image of each color, thereby deteriorating the color image.
[0004]
In order to solve this problem, a laser beam for writing an image on a photosensitive drum first among a plurality of laser beams in accordance with a phase relationship between an intermediate transfer reference signal and a line synchronization signal is switched for each color in the sub-scanning direction. There has been proposed a color image forming apparatus including a correcting unit for correcting a color shift by adjusting an image writing start position (see, for example, Patent Document 1).
However, in the above-mentioned invention, since the writing order of the laser beams is switched at random by the phase difference between the mark signal applied to the intermediate transfer belt and the synchronization detection signal, a plurality of laser beams are switched. In the case where there is a slight power deviation between the colors, the light energy applied to the photosensitive drum has a deviation for each color, even though the image data is the same, and the color balance is degraded. It is pointed out that it collapses (for example, see Patent Documents 2 and 3).
Experimentally, if there is a power deviation of about 2%, it is possible to visually observe the color shift. In particular, when halftone gray is reproduced, the adverse effect is remarkable, and the gray color may be colored instead of becoming gray for each copy.
[0005]
On the other hand, in the above-described color image forming apparatus, there is a demand for an improvement in recording speed, and in order to meet the demand, it is necessary to increase the rotation speed of a polygon mirror, which is a scanning unit of an optical scanning device, and increase the frequency of an image signal. is there. However, when the number of rotations of the scanning means and the frequency of the image signal are increased, the durability, noise, vibration, laser modulation speed, and the like of the motor for driving the polygon mirror are problematic and have limitations. Therefore, there has been proposed a multi-beam scanning apparatus that simultaneously scans a plurality of light beams and records a plurality of lines at the same time.
[0006]
As a method of a multi-beam light source device for emitting a plurality of laser beams, there is a method using a multi-beam semiconductor laser (for example, a semiconductor laser array) having a plurality of light-emitting points (light-emitting channels) in one package. However, it is difficult to increase the number of channels due to the manufacturing process, it is difficult to remove the influence of thermal / electrical crosstalk, and it is difficult to shorten the wavelength. Semiconductor lasers are recognized as expensive light source means.
[0007]
On the other hand, there is also a system using a single beam semiconductor laser as a light source, a light source device that combines a plurality of laser beams using a beam combining unit, and a multiple beam scanning device. When a plurality of laser beams are combined using the beam combining means, a problem arises in that the beam spot arrangement (beam pitch; scanning line interval) on the surface to be scanned fluctuates due to environmental fluctuations / aging fluctuations. Cheap. Therefore, a method of correcting a beam pitch using a liquid crystal element driven by an electric signal has been proposed.
[0008]
[Patent Document 1]
JP-A-10-239939
[Patent Document 2]
JP-A-2002-72606
[Patent Document 3]
JP-A-2002-72607
[0009]
[Problems to be solved by the invention]
However, as described above, when a liquid crystal element is used to adjust the position of a light beam on the surface to be scanned between a plurality of beams, a power deviation occurs between the plurality of beams, and a color image is generated due to the power deviation. The quality of the output image of the forming apparatus may be degraded.
[0010]
The present invention has been made in order to solve the problems of the conventional technology as described above, and it is possible to reduce a power deviation between a plurality of beams on a surface to be scanned and to suppress deterioration in quality of an output image. An object is to provide an optical scanning device and an image forming apparatus.
[0011]
[Means for Solving the Problems]
The invention according to claim 1 is an optical scanning device comprising a liquid crystal element for deflecting an optical path of a light beam from a light source to control a position of the light beam on a surface to be scanned, wherein the liquid crystal is associated with the optical path deflection by the liquid crystal element. It is characterized in that the transmittance variation T (%) of the element and the optical path deflection angle θ (minute) of the liquid crystal element satisfy the relationship of T / θ ≦ 2.0 (% / minute).
[0012]
According to a second aspect of the present invention, in the first aspect of the present invention, the liquid crystal element is a liquid crystal element in which an optical path deflection angle whose transmittance variation is 2.0 (% / min) or less periodically appears. It is characterized in that a period of transmittance variation appears in the deflection range of 10 or more.
[0013]
According to a third aspect of the present invention, in the first or second aspect of the present invention, there is further provided means for detecting the intensity of the light beam on the surface to be scanned.
[0014]
According to a fourth aspect of the present invention, in the first or second aspect of the present invention, there is further provided means for correcting the intensity of the light beam on the surface to be scanned.
[0015]
According to a fifth aspect of the present invention, there is provided an apparatus for performing optical writing from a light scanning device on a surface to be scanned and forming an electrostatic latent image on the surface to be scanned by an electrophotographic process. An optical scanning device according to any one of 1 to 4, wherein
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of an optical scanning device and an image forming apparatus according to the present invention will be described with reference to the drawings.
[0017]
FIG. 1 is an optical arrangement diagram in which an embodiment of the optical scanning device according to the present invention is developed in a plane parallel to a deflecting rotation plane. Reference numeral 41 denotes a first light source unit, reference numeral 42 denotes a second light source unit, reference numeral 17 denotes a beam combining prism, and reference numerals 40a and 40b deflect the optical paths of the light beams from the light sources 41 and 42, respectively. A liquid crystal element for controlling the position, 13 is a cylindrical lens, 14 is a polygon mirror which is a deflector, 15 is a scanning optical system, 16 is a photosensitive drum, 19 is a light beam intensity detecting means, 53 is an optical housing, and 54 is an optical housing. 4 shows a side wall of the housing. The polygon mirror 14 is driven to rotate at a constant speed by a drive mechanism having a motor (not shown).
Here, as an example of the optical scanning device used in the color image forming apparatus, a two-beam scanning device that simultaneously scans two light beams is shown, but the optical scanning device according to the present invention has a larger number of optical scanning devices. The present invention is applicable to a multi-beam scanning device that scans a light beam.
[0018]
The two light beams 21a and 21b emitted from the first light source unit 41 and the second light source unit 42 are combined by the beam combining prism 17, and then, by the action of the cylindrical lens 13, on the deflecting and reflecting surface of the polygon mirror 14, An image is formed in the sub-scanning direction, is formed as a long linear image in the main scanning direction, and is scanned by the scanning optical system (scanning lens) 15 as a beam spot on the surface of the photosensitive drum 16 which is the surface to be scanned. Here, the main scanning direction is a direction in which the beam spot is scanned on the surface of the photoconductor drum 16, and the sub-scanning direction is a direction orthogonal to the main scanning direction. The directions corresponding to the main scanning direction and the sub-scanning direction at each location are referred to as “main scanning direction” and “sub-scanning direction” in a broad sense, respectively.
[0019]
In the above-described optical scanning device (multi-beam scanning device), a light beam position correcting unit is provided for initial adjustment of the light beam position (light beam interval) on the surface to be scanned and correction of environmental / temporal variation. Often done.
FIG. 2 is an optical layout diagram showing an embodiment of the image forming apparatus according to the present invention.
The image forming apparatus 200 is a color image forming apparatus that outputs a color image, and four optical scanning apparatuses 20K, 20C, 20M, and 20Y similar to the optical scanning apparatus 20 shown in FIG. 1 are used as optical writing apparatuses. This is a so-called tandem image forming apparatus. Tandem-type image forming apparatuses are often provided with a light beam position correcting means for aligning a light beam between optical writing devices (stations). Details of the image forming apparatus 200 will be described later.
[0020]
Conventionally, as a basic configuration of the light beam position correcting means,
(1) The folding mirror is rotated
(2) Shift or rotate the cylindrical lens
(3) Shift or rotate the prism
(4) Use an electro-optical element or AOM
(5) Rotating a parallel plate disposed between the semiconductor laser and the coupling lens
A configuration capable of deflecting a light beam has been devised. However, in the conventional method, there are problems such as an increase in the size of the device or a large power consumption / heat generation / noise. Therefore, in the optical scanning device according to the present invention, as a light beam position correcting means, a "liquid crystal element" having features such as being silent and capable of responding to demands for small size, light weight, and energy saving is adopted. .
[0021]
By the way, when a liquid crystal element is used as the light beam position correcting means, the transmittance of the liquid crystal fluctuates according to the light beam position correction amount, that is, the beam deflection angle, as described later. The fluctuation of the transmittance of the liquid crystal may cause the fluctuation of the light power in the photosensitive drum 16, and when such an optical scanning device is used as an optical writing device of an image forming apparatus, the fluctuation of the light power may be reduced. Thus, there is a possibility that the output image is deteriorated. Therefore, in the present invention, the rate of change of the light beam intensity on the surface to be scanned is suppressed by suppressing the transmittance variation of the liquid crystal element due to the driving of the liquid crystal element to 4% or less, preferably 2% or less. Quality deterioration can be suppressed.
The “change rate of light beam intensity on the surface to be scanned” means “change rate of light beam intensity at the same image height”, and “shading characteristics” between image heights, that is, rotation of polygon mirror Does not include the variation in the reflectance due to the above and the deviation between the image heights of the transmittance / reflectance in the scanning lens, the folding mirror and the like.
[0022]
here,
Focal length of coupling lens: fcol = 15 (mm)
Sub-scanning imaging magnification of the whole multi-beam scanner: mZ = 9.5
Then
Optical path deflection angle in liquid crystal element: θ = 2.0 (min) = 0.66 (mrad)
The light beam position fluctuation amount Z on the scanned surface at the time of

It is. In the case of an optical system having the above conditions (fcol = 15 (mm), mZ = 9.5), changes in the position of the light beam on the surface to be scanned, that is, fluctuations with time, environmental fluctuations, fluctuations at the time of machine installation, etc., are experienced. Since it is about several tens of micrometers (μm), the optical path deflection angle for correcting the change in the light beam position is at most θ = 2.0 (minutes), and there is no problem.
[0023]
The variation of the transmittance of the liquid crystal element within the adjustment range of the optical path deflection angle is a very important parameter for maintaining the light beam position accuracy and the light beam intensity at the same time. That is, the transmittance variation T (%) of the liquid crystal element due to the optical path deflection by the liquid crystal element and the optical path deflection angle θ (minute) by the liquid crystal element are: T / θ = 4 (%) / 2.0 (minute) = 2.0 (% / min) or less, preferably 1.0 (% / min) or less, it is possible to effectively suppress a change in light beam intensity within a required light beam position adjustment range. It becomes.
[0024]
The following factors may cause variations in the light beam intensity on the surface to be scanned due to variations in the transmittance of the liquid crystal element.
(1) In a Matte-type liquid crystal element, a refractive index gradient is formed in a liquid crystal layer by changing the anisotropy Δn (= ne−no) of the refractive index of an ordinary ray and an extraordinary ray by a driving voltage applied to the liquid crystal. And then deflect the beam. Therefore, according to the beam deflection angle, that is, the anisotropy of the refractive index, the transmittance periodically changes as shown in FIG. In order to change Δn, a voltage is generally applied from the transparent electrode to a liquid crystal layer sandwiched between a pair of glass substrates, a transparent electrode, an alignment film, and the like. This makes it possible to generate a refractive index gradient proportional to the applied voltage.
(2) In the case of a configuration in which the beam is deflected by using the diffraction caused by the electrode pattern of the liquid crystal element, the transmittance changes because the diffraction efficiency changes according to the pitch of the diffraction grating.
[0025]
FIG. 3 is a diagram showing an example of a change in transmittance of the liquid crystal element due to beam deflection (adjustment of the position of the light beam on the surface to be scanned). The horizontal axis represents anisotropy Δn (= ne−) of the refractive index of the liquid crystal. The function u = 2 × Δn × d × λ, which is a parameter corresponding to the deflection angle φ of the light beam by the liquid crystal element. Here, d is the thickness of the liquid crystal layer, and λ is the wavelength of the light beam. In the case of an optical system in which the design constants constituting the liquid crystal element are appropriately set and the above conditions (fcol = 15 (mm), mZ = 9.5), the fluctuation range of the transmittance η of the liquid crystal is 4% (preferably 2%) may be used as the use range. That is, in the lower diagram of FIG. 3, a region A where the transmittance is 98% to 100% (width: 2%) is a use range of the liquid crystal element. However, in the region B in the upper part of FIG. 3, the wavefront aberration (spherical aberration) increases and the optical performance deteriorates, so that it cannot be used as an optical path deflecting element. Here, the transmittance η of the liquid crystal is
η = 1− [sin2 {(π / 2) × (1 + u2) 1/2}] / (1 + u2)
Is represented by
[0026]
In order to shift the light beam position on the scanned surface in the sub-scanning direction by Δz,
Δz = fcol × mZ × tanφZ
Than
φZ = tan-1 (Δz / fcol × mZ)
fcol: focal length of the coupling lens
mY: Main scanning magnification of the whole system
mZ: Sub-scanning magnification of the whole system
φY: Main scanning direction component of beam deflection angle
φZ: sub-scanning direction component of beam deflection angle
FY: main scanning focal length of the optical system after the deflector
So that
Sub-scanning direction component of beam deflection angle: φZ
Should be generated.
Also, to shift the beam spot position by Δy in the main scanning direction,
φY = tan-1 (Δy / fcol × mY) = tan-1 (Δy / FY)
So that
Main scanning direction component of beam deflection angle: φY
Should be generated.
[0027]
FIG. 4 is a diagram showing another example of the transmittance variation of the liquid crystal element due to the beam deflection.
In the case of the transmittance variation shown in FIG. 4, the region where the transmittance variation is 2%, that is, the regions A1 to An where the transmittance is 91% to 93% (width: 2%) are the liquid crystal element. The range of use. Since each of the regions A1 to An is discrete, a usable optical path deflection angle is also discrete. In such a case, a desired deflection angle can be obtained with high resolution by appropriately setting the design constant and shortening the cycle of vibration. However, since the wavefront aberration in the liquid crystal increases as the beam deflection angle, that is, the horizontal axis in FIG. 4 increases, for example, a region on the right side of the region An + 1 in FIG. 4 cannot be used as a light beam position adjusting unit. Area.
[0028]
In the optical scanning device according to the present invention, the period (cycle) of the periodic transmittance fluctuation generated due to the driving of the liquid crystal (adjustment of the light beam position by the light path deflection) is the light path deflection range, that is, the light beam position. 10 or more may appear in the adjustment range. For example, when it is desired to move the light beam position on the scanned surface by 95 (μm) in an optical system with the above condition (fcol = 15 (mm), mZ = 9.5), that is, when the adjustment range is 95 μm By setting the period of the transmittance variation to 10 or more, the light beam position can be adjusted with a resolution of at least 9.5 (μm).
[0029]
Further, if the maximum value in each cycle is made substantially constant, the transmittance can be maintained on a high side, and an increase in energy loss can be avoided.
Furthermore, as shown in FIG. 5, when the transmittance (vertical axis) tends to decrease while periodically oscillating with an increase in Δn (horizontal axis), within a necessary beam deflection angle range, A region where the transmittance variation is 4% (preferably 2%), that is, a region A where the transmittance is 94% to 96% (width: 2%) in the example of FIG. 5 is a use range of the liquid crystal element. .
[0030]
As shown in the upper diagram of FIG. 3, generally, the optical path deflection angle linearly increases with an increase in the voltage applied to the liquid crystal layer, that is, with an increase in Δn, but when the applied voltage exceeds a certain value, this linearity is lost. In addition, problems such as an increase in wavefront aberration of the liquid crystal layer occur (see the region B in the upper diagram of FIG. 3).
In addition, since the region on the left side of the drawing from the linear region in the upper drawing of FIG. 3 is nonlinear, the use as an optical path deflecting element becomes complicated. In consideration of this point, it is necessary to design so that the “region where the transmittance variation is 2% or less” and the “region where the optical path deflection angle shows linearity” are wide.
[0031]
According to the embodiment described above, the variation in the transmittance in the optical path deflection angle range (light beam position adjustment range) of the liquid crystal element is suppressed, so that the deviation of the light beam intensity at the surface to be scanned, that is, at the same image height. And / or the deviation between the multiple beams can be reduced.
Therefore, it is possible to effectively suppress the deterioration of the color image quality due to the shift in the balance between the colors that occurs when the writing light beam is switched.
[0032]
In the optical scanning device according to the present invention, the light beam intensity detecting means detects the light beam intensity on the surface to be scanned or a surface equivalent to the surface to be scanned, and when the light beam intensity exceeds a predetermined value. The liquid crystal element may be driven, that is, the optical path may be deflected so that the light beam intensity falls within a predetermined value. When the transmittance characteristic as shown in FIG. 4 is exhibited, the adjustment value of the light beam position may be discrete, but if the period of the transmittance oscillation is appropriately designed according to the resolution of the light beam position adjustment. Good.
[0033]
Further, the deviation of the light beam intensity on the scanned surface and / or the deviation of the light beam intensity among a plurality of beams is further improved by the light beam intensity correcting means for correcting the intensity of the light beam on the scanned surface (for example, 1% or less). ).
As shown in the upper diagram of FIG. 3, the relationship between the “optical path deflection angle in the liquid crystal element” and the “fluctuation amount of the light beam intensity on the surface to be scanned” is generally determined theoretically (design). Therefore, it is preferable to prepare a table for calculating the light power correction amount from the beam deflection amount, and drive / control the “light beam intensity correction means” based on the table (not shown). In this case, it is not necessary to provide “light beam intensity detection means”.
[0034]
As an example of the light beam intensity correction means, for example, the light emission output of a light source (semiconductor laser) can be changed. Alternatively, a filter, for example, a filter using the polarization characteristics of the light beam, a filter that simply absorbs light energy inside, or reflects light on the surface, or the like may be arranged in the light path of the light beam.
[0035]
However, the theoretical value (design value) may fluctuate due to component variation / assembly variation, deterioration over time, environmental (temperature, humidity) change, and the like in an actual machine. Therefore, as shown in FIG. 1, the light beam intensity detecting means 19 is provided on the surface to be scanned (or an optically equivalent position), and the light beam intensity correcting means is driven based on the detection result of the light beam intensity detecting means 19. Or control.
[0036]
Next, an image forming apparatus according to the present invention will be described.
FIG. 2 is an optical layout diagram showing an embodiment of the image forming apparatus according to the present invention.
The image forming apparatus 200 includes optical scanning devices 20K, 20, C, 20M, and 20Y corresponding to black (K), cyan (C), magenta (M), and yellow (Y), and photosensitive drums 16K, 16C, Electrophotographic processes such as 16M, 16Y, developing means for developing the electrostatic latent image formed on the surface of the photosensitive drum with toner, and transfer means for transferring the visualized toner image to recording paper. Is a tandem-type image forming apparatus which is advantageous for increasing the output speed / density of a color image by simultaneously scanning a plurality of beams. The surface of each photosensitive drum is the surface to be scanned of the optical scanning device. In a color image forming apparatus having four such photosensitive units, an output image that is four times as large as an image forming apparatus that requires one photosensitive unit, that is, four writings corresponding to four colors, is required. It is possible to obtain. In the drawings, illustrations of a developing unit, a transfer unit, and the like are omitted.
[0037]
Around the photosensitive drums, process members according to an electrophotographic process, such as a charger, a developing device, and a transfer device (not shown), are sequentially arranged. The optical scanning device executes an exposure process of an electrophotographic process, and forms an electrostatic latent image by scanning the surface of a photosensitive drum uniformly charged by a charger.
[0038]
In such a tandem-type image forming apparatus having a plurality of photoconductors, for example, when the multi-color mode is selected, the photoconductor drums 16K, 16C, 16M, and 16Y are illustrated according to the corresponding color image signals. By the exposure of the unexposed exposure unit, an electrostatic latent image is formed on the photosensitive drums 16K, 16C, 16M, and 16Y. These electrostatic latent images are developed with corresponding color toners to form toner images, electrostatically attracted to a transfer belt, and sequentially transferred onto a transfer paper to be conveyed, thereby being superimposed. Then, the image is fixed as a color image by the fixing device and is discharged. The illustration of the transfer belt, the transfer paper, and the fixing device is omitted.
When the monochrome mode is selected, only the photosensitive drum and the process member corresponding to a certain color (one of K, C, M, and Y) are operated, and the photosensitive drum and the process member of the other color are not operated. It becomes an operation state.
[0039]
In the image forming apparatus 200, when the number of beams emitted from all the optical scanning devices 20K, 20C, 20M, and 20Y is one, a full-color (four-color) image can be obtained. On the other hand, when at least one of the four optical scanning devices (for example, the optical scanning device 20K corresponding to black) is the optical scanning device of plural (N) beams according to the present invention, and the optical scanning is performed only by this optical scanning device, It is possible to increase the density by a factor of N compared to a full-color image.
Further, if the transport speed (and the process speed) of the recording medium is quadrupled, the number of output images can be quadrupled. Further, even in the case of a full-color image, since a character image is often written in black and high resolution is often required, it is added to the above-mentioned multiple (N) beam light scanning device 20K (black). By simultaneously writing the optical scanning devices (20C, 20M, 20Y; one beam), it is possible to obtain a higher quality output image even in an image in which characters / photos / line drawing images are mixed.
[0040]
By applying the optical scanning device according to the present invention to the optical scanning device corresponding to each color included in the image forming apparatus 200, the relative position between a plurality of beams that scan the surface of one photosensitive drum, that is, the surface to be scanned, that is, The beam pitch can be adjusted to a desired value, and a high-quality output image can be obtained.
[0041]
FIG. 7 is an optical layout diagram showing another embodiment of the image forming apparatus according to the present invention. This image forming apparatus is a tandem-type image forming apparatus in which a plurality of light beams emitted from a light source unit including a plurality of light sources scan on a plurality of scanned surfaces simultaneously. The configuration of the polygon mirror as the deflecting means, the configuration of the scanning optical system, and the like are the same as those of the known means, and therefore, the description thereof is omitted. In the optical path of the light beam reaching the photosensitive drums 16K, 16C, 16M and 16Y, a liquid crystal element 40 for adjusting the position of the light beam on each photosensitive drum is provided. The liquid crystal element 40 may have a plurality of effective areas in one element, or may be an independent element having one effective area for each light beam.
[0042]
If the optical scanning device according to the present invention is applied to this image forming apparatus, the liquid crystal element 40 can correct the relative light beam position between the plurality of photosensitive drums. That is, for example, by detecting the toner image 32 formed on the transfer belt 31 with the color misregistration detection sensor 33 and driving the liquid crystal element 40 based on the detection result, that is, the degree of color misregistration between stations, The writing start timing (that is, the writing start position) of each station in the sub-scanning direction (or in the main scanning direction) can be corrected. Therefore, color shift of the toner image 32 on the transfer belt 31 can be reduced, and a high quality output image can be obtained.
[0043]
Here, when several tens or more sheets are continuously printed by a copying machine, a printer, or the like, there is a possibility that a color shift between stations may occur due to an influence of a temperature rise in the machine. When the above-described liquid crystal element is used to correct such a color shift, there is a possibility that the transmittance varies, that is, the light beam intensity varies on the photosensitive drum according to the correction amount. However, if the optical scanning device according to the present invention is used, since the fluctuation of the light beam intensity can be suppressed, it is possible to suppress the deterioration of the output image quality even during continuous printing.
[0044]
In addition, it is possible to use it not only for the fluctuation at the time of continuous printing, but also for the correction at the time of machine movement / installation, the fluctuation over time, and the like.
Further, the liquid crystal element does not need to be provided in the optical paths of all the light beams. In order to perform alignment with respect to a certain reference color (for example, black), a liquid crystal element may be provided only in the optical path of another color (for example, cyan, magenta, and yellow).
[0045]
FIG. 6 is an optical arrangement diagram showing a still another embodiment of the image forming apparatus according to the present invention, which is developed in a plane parallel to the deflecting rotation plane. In this image forming apparatus, a plurality of optical scanning devices 20 are arranged in parallel in the main scanning direction, and the effective writing width is divided so as to scan on the scanned surface 16.
If the optical scanning device according to the present invention is applied to this image forming apparatus, it is possible to correct the light beam position (at the time of initial adjustment, environment / temporal variation, etc.) at the joint of the divided scans on the scanned surface 16. If the effective writing width can be increased, the optical elements and deflectors constituting the optical scanning device can be reduced in size, so that fluctuations in the beam waist position due to mechanical tolerances and temperature fluctuations are reduced, and wavefront aberration is reduced. As a result, a high quality output image can be obtained.
[0046]
【The invention's effect】
According to the present invention, the transmittance variation in the optical path deflection angle range of the liquid crystal element that controls the position of the light beam on the surface to be scanned by deflecting the optical path of the light beam from the light source is suppressed. Power deviation between a plurality of beams can be reduced, and deterioration of the quality of an output image can be suppressed.
[Brief description of the drawings]
FIG. 1 is an optical layout diagram showing an embodiment of an optical scanning device according to the present invention, which is developed in a plane parallel to a deflecting and reflecting surface.
FIG. 2 is an optical layout diagram showing an embodiment of the image forming apparatus according to the present invention.
FIG. 3 is a diagram illustrating an example of a change in transmittance of a liquid crystal element due to beam deflection.
FIG. 4 is a diagram showing another example of a change in transmittance of a liquid crystal element due to beam deflection.
FIG. 5 is a diagram showing still another example of the transmittance fluctuation of the liquid crystal element due to the beam deflection.
FIG. 6 is an optical layout diagram showing another embodiment of the image forming apparatus according to the present invention.
FIG. 7 is an optical layout diagram showing still another embodiment of the image forming apparatus according to the present invention.
[Explanation of symbols]
13 Cylindrical lens
14 Polygon mirror
15 Scanning optical system
16 Photoconductor drum
19 Light beam intensity detecting means
20 Optical scanning device
40a, 40b liquid crystal element
41, 42 light source unit

Claims (5)

An optical scanning device including a liquid crystal element that controls a light beam position on a surface to be scanned by deflecting an optical path of a light beam from a light source,
The transmittance variation T (%) of the liquid crystal element due to the light path deflection by the liquid crystal element and the light path deflection angle θ (minute) by the liquid crystal element satisfy the relationship of T / θ ≦ 2.0 (% / minute). An optical scanning device, comprising: 2. The liquid crystal device according to claim 1, wherein the light path deflection angle at which the transmittance variation is 2.0 (% / min) or less periodically appears, and the transmittance variation period appears at least 10 times in the optical path deflection range. Optical scanning device. 3. The optical scanning device according to claim 1, further comprising a unit configured to detect an intensity of the light beam on the surface to be scanned. 3. The optical scanning device according to claim 1, further comprising a unit that corrects the intensity of the light beam on the surface to be scanned. A device that performs optical writing on a scanned surface from an optical scanning device and forms an electrostatic latent image on the scanned surface by an electrophotographic process,
The image forming apparatus according to claim 1, wherein the optical scanning device is the optical scanning device according to claim 1.

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