JP2004106365A - Image formation apparatus and copying apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form high quality images faithful to original images by preventing generation of banding because of superposed exposure without being influenced by resolutions also when writing is carried out by a high resolution. <P>SOLUTION: In the image formation apparatus, a surface of a photoreceptor 13 charged by a charging device is set as a face to be scanned, and a multibeam scanning device 18 is installed for simultaneously light scanning on the face by a plurality of light beams obtained by deflecting and scanning each of light emitted from a plurality of light sources. Toner supplied to the surface of the photoreceptor 13 light scanned by the multibeam scanning device 18 is transferred to a body to be transferred. A superposition length in a vertical scanning direction of scanning lines formed on the face by the simultaneous light scanning by the multibeam scanning device 18 is set to be not larger than a minimum pixel. <P>COPYRIGHT: (C)2004,JPO

Description

【００６５】
（２）電荷キャリア生成とその輸送の過程
感光体１３として、ＯＰＣ（有機感光体）における電荷キャリア生成とその輸送の過程は、以下に示す（２）式、（３）式、（４）式により正負キャリアの連続の式とポアソン方程式によって支配される。
【００６６】
【数２】

【００６７】
ただし、ｎ：キャリアの個数密度、μ：キャリアの移動度、Ｅ：電界強度、Γ：単位時間あたりのキャリアの生成量、ｒ：キャリアの単位時間あたりの再結合係数、ε：誘電率、ｅ：電荷素量、ｐ：正キャリア、ｎ：正負キャリア
【００６８】
ここで、キャリア発生層が薄いため、電荷キャリアが層内で一様に生成されると仮定すると、キャリア生成量Γと、入射光強度Ｆ，量子効率η，キャリア発生層の厚さｄとの関係は、以下に示す（５）式によって表わされる。
【００６９】
【数３】

【００７０】
ただし、β：キャリア発生層内での光の吸収効率、η：量子効率、ｈυ：レーザビームのフォトン一個あたりのエネルギー
【００７１】
量子効率ηは、電界に依存し、η＝αＥ^ｎで表される。
【００７２】
なお、上記（２）式，（３）式の右辺第２項目は、正負キャリアが同じ近傍に共存する際に、実験的には生成キャリア量が減少することを説明するために導入されたキャリア再結合項である。
【００７３】
上記の物理量のうち、光の吸収係数βおよび再結合係数Ｒは、黒ベタ露光時の表面電位から実験的なフィッティングを行ない算出する。
【００７４】
そして、露光前の感光体１３表面が均一に帯電していると仮定して感光体１３表面の電荷量を算出した後、上記の（２）式、（３）式、（４）式を計算することにより、露光後の感光体１３表面における電荷密度分布を算出した。以降、露光後の感光体１３表面の電荷密度分布を、「感光体上静電潜像電荷分布」とする。
【００７５】
２．現像電界強度分布算出
次に、上述した演算により得られた感光体上静電潜像電荷分布と、現像条件とから現像電界強度分布を算出する。ここで、現像スリーブ１５ｂと感光体１３を平行平板で近似し、その間に現像剤（キャリアとトナー）が均一に充填していると仮定すると、この現像剤によって形成される層を、平均誘電率εをもった均一な誘電層と見なすことができる。この誘電層における平均誘電率εは、平均比誘電率ε’と真空の誘電率ε_０との積によって表わされる（（６）式参照）。このとき、平均比誘電率ε’は、以下に示す（７）式の演算により取得される。
【００７６】
【数４】

【００７７】
ただし、ε_１：キャリアの比誘電率、ａ：現像ニップ中でのキャリアの占める体積の割合
【００７８】
現像スリーブ１５ｂ表面が現像バイアス電位、感光体１３下面が０Ｖで一定の境界条件、左右の境界を周期境界条件として、ポアソン方程式を解くことで、感光体１３表面の電位分布が得られる。そして、得られた感光体１３表面の電位分布に基づいて、以下に示す（８）式を演算することにより感光体１３表面の現像電界強度分布を取得することができる。
【００７９】
【数５】

【００８０】
３．静電潜像ドット径および静電潜像断面積値の算出
感光体１３表面におけるトナー像は、感光体１３表面における現像電界強度分布にしたがって、ある閾値電界強度以上の領域にトナーが付着することにより形成される。本実施の形態では、この「ある閾値電界強度」を、臨界現像電界強度と定義し、現像プロセスや現像剤の条件によって決まる値とする。
【００８１】
ここで、図３は、静電潜像の副走査方向におけるライン径と電界強度との相関を示すグラフである。上述したように、現像時の電界強度分布において臨界現像電界強度以上の領域にトナーが付着するため、図３中ラインＥｔｈを臨界現像電界強度のラインとすると、トナーは、ラインＥｔｈ以上の電界強度を有する領域に付着する。本実施の形態では、静電潜像の副走査方向におけるライン径に対する電界強度について考慮しているため、図３中斜線で示すラインＥｔｈ以上の電界強度を有する領域にトナーが付着する。図３では、現像時の電界強度分布において臨界現像電界強度以上の領域の副走査方向の長さを、静電潜像の副走査方向におけるライン径とする。トナー付着量は、静電潜像の副走査方向におけるラインの断面積に相当する。
【００８２】
なお、本実施の形態では、静電潜像の副走査方向におけるライン径としたが、これに限るものではなく、静電潜像ドット径とする場合、主走査方向および副走査方向の長さを考慮する。
【００８３】
このようなことから、臨界現像電界強度が分かれば、感光体１３上のトナーが付着している領域の径、すなわち、静電潜像ドット径（ラインであればライン径）、および、静電潜像断面積値を算出することが可能である。
【００８４】
ここで、図４は、同時光走査によって主走査方向に沿って２ドットライン静電潜像を形成した場合のキャリア発生量と、順次走査によって主走査方向に沿って２ドットライン静電潜像を形成した場合のキャリア発生量との違いを示すグラフである。また、図５は、同時光走査によって静電潜像を形成した場合の電界強度分布と、順次走査によって同じ静電潜像を形成した場合の電界強度分布とを示すグラフである。
【００８５】
図４から判るように、順次露光時の２ライン目の光走査時におけるキャリアの発生量は、同時露光時の２ライン目の光走査時におけるキャリアの発生量よりも少ない。順次露光では１ライン目を光走査してから次の２ライン目を光走査するため、２ライン目を光走査時にすでにキャリアが発生しており、このために電界強度が弱まってキャリアの発生量が減少する。これに対し、同時露光時では、１ライン目と２ライン目とを同時に露光走査するため、キャリア生成量は減少せず、順次露光よりも同時露光時の方がキャリアの生成量が多くなって、露光が重なっている領域（網掛け部）のキャリア個数が多くなる。
【００８６】
キャリア再結合は、上述したように（（２）式、（３）式参照）キャリア生成量に依存するため、キャリアの生成量が多い同時露光時の方が順次露光時よりもキャリアの再結合が多く発生する。
【００８７】
このため、同時露光時の方が順次露光時よりもキャリア発生量が多いにもかかわらず、図５に示すように、同時露光時の方が順次露光時の電界強度分布より狭く小さくなるという相反則不規現象が発生する。
【００８８】
このように相反則不規現象が発生している静電潜像をトナーで顕像化することにより、ラインの幅の変動が周期的に発生し、バンディングとして観察されるのである。
【００８９】
高画質化に伴って書き込みが高密度になればなる程、生成キャリア数が増加し、キャリアの再結合の影響が大きくなるため、書き込みが高密度になればなる程、バンディング現象が顕著に発生することが懸念される。
【００９０】
ここで、図６は、書込密度１２００ｄｐｉのマルチビーム走査方式で、同時露光と順次露光との２方式により２ドット主走査ラインを露光することによって静電潜像を形成した場合の同時露光によるライン断面積と順次露光によるライン断面積との差（ライン断面積差）と知覚バンディングとの関係を示すグラフである。
【００９１】
知覚バンディングとは、バンディングを定量的に評価する評価尺度であり、知覚されるバンディングの大きさを画像の平均明度Ｌと空間周波数ｆと空間周波数における明度振幅ΔＬの関数として、以下に示す（９）式の演算によって算出される。
【００９２】
【数６】

【００９３】
ただし、ａ，ｂ，ｃ，ｄ，ｅ，ｇ：定数、Ｌ：平均明度、ｆ：空間周波数、ΔＬ：空間周波数における明度振幅
図６から判るように、バンディングの影響は、ライン断面積差の減少に伴って小さくなる。
【００９４】
ところで、バンディングとして知覚されない知覚バンディング値は０．２以下とされており、図６によれば、知覚バンディング値が０．２以下となるライン断面積差は、２．７×１０^−６以下であることが判る。
【００９５】
すなわち、図６によれば、ライン断面積差を２．７×１０^−６以下に抑えることにより、バンディングを防止することができることが判る。
【００９６】
ここで、図７は、副走査方向のビーム重畳長さとライン断面積差との関係を示すグラフである。図７中、副走査方向のビーム重畳長さは、ビームスポット径が重なる部分の副走査方向の長さを示している。なお、ビームスポット径は、ビーム最大強度の１／ｅ^２径で表わされる。
【００９７】
重畳長さが減少すると、重畳露光部分が減少する。上述したように、重畳露光部分が減少するとキャリア生成量が減少する。このため、副走査方向のビーム重畳長さを減少させることにより、同時露光時のキャリア再結合が起こりにくくなってライン断面積差が低下すると考えられる。
【００９８】
図６によれば、バンディングの発生が抑制される断面積差は２．７×１０^−６μｍ^２であるため、図７より、副走査方向における重畳長さを２１μｍ以下とすることによりバンディングの発生を抑制することができる。
【００９９】
ところで、重畳長さが最小画素以下であれば、重畳部分があったとしてもその重畳部分は、バンディングとして知覚されない。
【０１００】
本実施の形態のレーザプリンタ１は、目的とする解像度を１２００ｄｐｉとしているため、この解像度において解像可能な最小画素の大きさは、（１０）式の演算によって求められる。
【０１０１】
【数７】

【０１０２】
（１０）式によれば、書き込み解像度が１２００ｄｐｉである場合の解像可能な最小画素の大きさが２１．１７×１０^−６μｍであることが判る。すなわち、副走査方向の重畳長さを２１．１７×１０^−６μｍ以下とすることにより、バンディングの発生を抑制することができる。
【０１０３】
次に、本発明の第２の実施の形態について図８および図９を参照して説明する。なお、第１の実施の形態と同一部分は同一符号で示し、説明も省略する。
【０１０４】
本実施の形態のレーザプリンタ１における半導体レーザアレイ１９のチャンネル数は４であり、各発光部ｃｈ１，ｃｈ２，ｃｈ３，ｃｈ４から発光する光の発光波長は７８０ｎｍに設定されている。このような半導体レーザアレイ１９を備えるマルチビーム走査装置１８は、感光体１３表面において３５〜７０μｍのビームスポットを照射し、レーザースキャン速度１９８３．８７ｍ／ｓ（画周波数：９２．５ＭＨｚ）、線速０．３６２ｍ／ｓ（走査周波数４２７５．６Ｈｚ）、書き込み密度１２００ｄｐｉで光走査（露光）を行なう。露光に際しての必要露光エネルギーは、７ｅｒｇ／ｃｍ^２である。
【０１０５】
感光体１３の量子効率ηは、５．６×１０^−６・Ｅ^{０．７０２２}であり、感光体１３の吸収係数は０．６５、感光体１３の膜厚は３０μｍに設定されている。
【０１０６】
加えて、帯電ローラ１４による感光体１３表面の初期帯電電位は、−８００Ｖに設定されている。
【０１０７】
ここで、図８は、本実施の形態のレーザプリンタ１において、ビームスポットの副走査方向の径と、静電潜像ラインの副走査方向での重畳長さとの関係を示すグラフである。図８によれば、ビームスポットの副走査方向における径を４２μｍ以下とすることにより、第１の実施の形態と同様に、副走査方向の重畳長さを最小画素（１画素）以下とすることができる。
【０１０８】
ところで、図８からも判るように、副走査方向のビーム重畳長さは、ビームスポット径の小径化に伴って減少する。
【０１０９】
ここで、図９は、ビームスポットの副走査方向における径とライン幅との関係を示すグラフである。図９から判るように、ビームスポットの副走査方向における径が４２μｍ以下になると、同時露光と順次露光でのライン幅がほぼ一致する。これによって、書き込み解像度が１２００ｄｐｉであるレーザプリンタ１では、ビームスポットの副走査方向における径を４２μｍ以下とすることによりバンディングの発生を抑制することができる。
【０１１０】
次に、本発明の第３の実施の形態について図１０を参照して説明する。
【０１１１】
本実施の形態のレーザプリンタ１が備えるマルチビーム走査装置１８の半導体レーザアレイ１９におけるチャンネル数は２であり、各発光部ｃｈ１，ｃｈ２から発光する光の発光波長は６５５ｎｍに設定されている。
【０１１２】
このような半導体レーザアレイ１９を備えるマルチビーム走査装置１８は、感光体１３表面において主走査方向に７０μｍ，副走査方向に８５μｍのビームスポットを照射し、画周波数６４．３ＭＨｚ、線速７９ｍｍ／ｓ、書き込み密度１２００ｄｐｉで光走査を行なう。露光に際しての必要露光エネルギーは、７ｅｒｇ／ｃｍ^２である。
【０１１３】
マルチビーム走査装置１８によって光走査される感光体１３表面における副走査方向のビーム重畳長さは、１画素以下に設定されている。
【０１１４】
加えて、本実施の形態の感光体１３の量子効率ηは、２．８３×１０^−５・Ｅ^{０．５９３３}であり、感光体１３の吸収係数は０．７、感光体１３の膜厚は２８μｍに設定されている。
【０１１５】
ところで、感光体１３の帯電電位とキャリア移動層の厚さ（膜厚）とから求められる電界強度（帯電電位／キャリア移動層厚）が３０Ｖ／μｍよりも大きくなると、地汚れが発生しやすくなるといわれている。そして、このような地汚れを防止して良好な画像を形成するための感光体１３の量子効率ηは０．４＜η＜１とされている。本実施の形態の感光体１３の量子効率ηは、２．８３×１０^−５・Ｅ^{０．５９３３}であるため、上述の０．４＜η＜１の範囲に設定されている。
【０１１６】
ここで、図１０は、該感光体１３における光減衰特性を示すグラフである。図１０から判るように、感光体１３に対してマルチビームで順次走査で書き込みした時の減衰特性と同時光走査で書き込みした時の減衰特性とは、異なる曲線になっている。順次光走査による書き込み時の減衰特性と同時光走査による書き込み時の減衰特性との違いは、静電潜像のドット幅やライン幅の差を発生させ、バンディングの原因となっている。
【０１１７】
本実施の形態のレーザプリンタ１は、副走査方向のビーム重畳長さが１画素以下に設定されたマルチビーム走査装置１８を用いているため、量子効率ηが０．４＜η＜１の範囲に設定された感光体１３を用いる場合にも、同時光走査時の静電潜像電荷減少を防ぎ、バンディングの発生を抑制することができる。
【０１１８】
次に、本発明の第４の実施の形態について図１１を参照して説明する。
【０１１９】
本実施の形態のレーザプリンタ１は、入出力特性を直線回帰した場合の相関係数Ｒ^２値が０．９７以上を達成するように設定された入出力特性を有している。
【０１２０】
また、マルチビーム走査装置１８は、感光体１３表面での副走査方向の重畳長さが、最小画素以下となるビームを照射する。本実施の形態のマルチビーム走査装置１８の書き込み密度は１２００ｄｐｉであり、このため、感光体１３表面での副走査方向の重畳長さは２１．１７μｍ（１画素）以下となるように設定されている。
【０１２１】
ところで、レーザプリンタ１における高解像度化や階調再現性の向上は、ベタ濃度、入出力特性の滑らかさ、階調のとびやつぶれ等の条件によって決定される。
【０１２２】
すなわち、十分なベタ濃度があって、レーザプリンタ１における入出力特性が滑らかであり階調のとびやつぶれが少ないレーザプリンタ１では、高解像度で階調再現性のよい画像形成を実現することができる。
【０１２３】
図１１は、このような高解像かつ高画質な画像を形成するレーザプリンタ１の入出力特性の一例を示すグラフである。図１１中点線で示すように、レーザプリンタ１の入出力特性は、入出力特性の直線回帰時の相関係数Ｒ^２値が低い時は、マルチビームによって形成された静電潜像ライン幅の差の影響を受けないが、相関係数が高くなってくるとライン幅の差による影響が大きくなってくる。図１１に示す入出力特性では、相関係数Ｒ^２値が０．９７以上になると、ライン幅の差による影響が大きくなってくる。
【０１２４】
本実施の形態では、書き込み密度１２００ｄｐｉを実現するマルチビーム走査装置１８において、感光体１３表面での副走査方向の重畳長さを最小画素以下、すなわち、２１．１７μｍ以下とすることにより、相関係数Ｒ^２値が０．９７以上を達成するレーザプリンタ１においても、バンディングを抑制することができる。
【０１２５】
次に、本発明の第５の実施の形態について図１２および図１３を参照して説明する。
【０１２６】
本実施の形態のレーザプリンタ１が備えるマルチビーム走査装置１８の半導体レーザアレイ１９におけるチャンネル数は２である。
【０１２７】
本実施の形態の現像装置１５のケース１５ａ内には、粒径６．５μｍのトナーが保持されている。ケース１５ａ内のトナーの着色剤含有量（トナーの着色度）は、５〜１５ｗｔ％の範囲内に設定されている。本実施の形態では、トナーの着色剤含有量が、６％になるように設定されている。
【０１２８】
現像装置１５は、臨界電界強度以上の領域（ソリッド領域）のトナー付着量Ｍ／Ａが、０．２≦Ｍ／Ａ≦０．８ｍｇ／ｃｍ^２を満足する範囲内となるように現像を行なう。ソリッド領域のトナー付着量Ｍ／Ａを上述した範囲内として現像を行なうことにより、感光体表面には、一層分のトナーが付着していることになる。以降、このような条件での現像を、低Ｍ／Ａ現像とする。本実施の形態では、ソリッド領域のトナー付着量Ｍ／Ａが０．４７ｍｇ／ｃｍ^２を満たすように現像が行われる。
【０１２９】
低Ｍ／Ａ現像を行なうことによる感光体１３上のトナー付着量は、臨界現像電界強度で切ったときの静電潜像ライン幅に相当する。
【０１３０】
ここで、図１２は、副走査方向における静電潜像ライン幅と知覚バンディングとの関係を示すグラフである。図１２中、同時露光時の静電ライン幅と順次露光時のライン幅との差（ライン幅差）をΔライン幅として示している。図１２によれば、同時露光時の静電潜像ライン幅と順次露光時の静電潜像のライン幅との差（ライン幅差）が、４．２μｍである場合にバンディングが発生しないことが判る。ここで、半導体レーザアレイ１９におけるチャンネル数が２であるため、画像のライン幅は、４２μｍ程度（２１．１７μｍ×２）であり、４．２μｍとは画像ライン幅の約１割に相当することが判る。
【０１３１】
図１３は、同時露光時の静電ライン幅と順次露光時のライン幅との差（ライン幅差）と、副走査方向の重畳長さとの関係を示すグラフである。図１３によれば、低Ｍ／Ａ現像を採用したレーザプリンタ１では、バンディングが発生しない副走査方向の重畳長さは、４２μｍ以下であることが判る。
【０１３２】
このように、低Ｍ／Ａ現像を採用したレーザプリンタ１では、重畳長さが４２μｍ以下であればバンディングを抑制することができ、低Ｍ／Ａ現像を採用していないレーザプリンタと比較してバンディングを低減するためのビーム重畳長さの許容値が広くなる。これにより、低Ｍ／Ａ現像を採用していないレーザプリンタと比較して、ビーム重畳長さの管理を容易にすることができる。
【０１３３】
ところで、重畳長さ４２μｍという数値は、解像度１２００ｄｐｉにおいて解像可能な最小画素のほぼ２倍であることが判る。すなわち、低Ｍ／Ａ現像を採用することにより、バンディングを低減するためのビーム重畳長さの許容値を、最小画素の２倍まで広げることができる。
【０１３４】
次に、本発明の第６の実施の形態について図１４を参照して説明する。本実施の形態は、第５の実施の形態のレーザプリンタと同様のレーザプリンタ１である。
【０１３５】
図１４は、本実施の形態のレーザプリンタ１におけるビームスポット径と副走査方向における重畳長さとの関係を示すグラフである。図１４によれば、書き込み密度１２００ｄｐｉのマルチビーム走査装置１８で、解像可能な最小画素の２倍である２１．１７×２＝４２．３３μｍ以下を達成するための副走査方向におけるビームスポット径は、６３μｍ以下であることが判る。
【０１３６】
第２の実施の形態で説明したように、低Ｍ／Ａ現像を採用していないレーザプリンタ１で書き込み密度１２００ｄｐｉで光走査を行なう場合には、副走査方向におけるビームスポット径を４２μｍ以下とする必要があるが、低Ｍ／Ａ現像を採用することにより、バンディングを低減するための副走査方向におけるビームスポット径の許容値を６３μｍまで広げることができる。これにより、低Ｍ／Ａ現像を採用していないレーザプリンタと比較して、ビーム重畳長さの管理を容易にすることができる。
【０１３７】
次に、本発明の第７の実施の形態について図１５を参照して説明する。
【０１３８】
本実施の形態では、副走査方向のビームの重畳長さの調整に代えて、順次露光時の露光量を弱めることにより、順次露光時のライン幅と同時露光時のライン幅との差を低減させる。
【０１３９】
図１５は、本実施の形態のレーザプリンタ１で、順次露光部分の露光量を１００％として露光を行なった場合の露光量分布と、順次露光部分の露光量を９０％として露光を行なった場合の露光量分布とを示すグラフである。本実施の形態では、半導体レーザアレイ１９の、副走査方向において両端となる発光部ｃｈ１，ｃｈ４の発光量を中央の発光部ｃｈ２，ｃｈ３と比較して低減することにより、順次露光部分の露光量を調整した。図１５から判るように、発光部ｃｈ１，ｃｈ４の発光量を中央の発光部ｃｈ２，ｃｈ３に対して９０％（露光量９０％）として露光を行なった場合の副走査方向のビーム重畳長さは、発光部ｃｈ１，ｃｈ４の発光量を中央の発光部ｃｈ２，ｃｈ３に対して１００％（露光量１００％）として順次露光を行なった場合の副走査方向のビーム重畳長さと変わっていないものの、露光量９０％で露光を行なった場合の重畳部における露光量（重畳露光量）は、露光量１００％で順次露光を行なった場合の重畳露光量と比較して低下していることが判る。
【０１４０】
ここで、図１６は、重畳露光量と知覚バンディングとの関係を示すグラフである。図１６から判るように、順次露光時の重畳露光量を低減させることにより知覚バンディングが減少することが判る。
【０１４１】
図１６によれば、露光量を低下させ重畳露光量を低減させることにより知覚バンディングの抑制が期待される。
【０１４２】
しかしながら、露光量を弱めすぎると、順次露光による静電潜像ライン幅が同時露光による静電潜像ライン幅よりも小さくなってしまい、これによって順次露光時のライン幅と同時露光時のライン幅との差が大きくなり、バンディングとして知覚されることが懸念される。
【０１４３】
つまり、順次露光時の露光量の変化による知覚バンディングの変化を把握し、重畳露光によってバンディング現象が発生しないように露光量を制御する必要がある。
【０１４４】
ここで、図１７は、順次露光時の露光量の変化による知覚バンディングの変化の関係を示すグラフである。図１７から判るように、発光部ｃｈ１，ｃｈ４の発光量が中央の発光部ｃｈ２，ｃｈ３に対して９０％（露光量９０％）となるように各発光部ｃｈ１，ｃｈ２，ｃｈ３，ｃｈ４からの発光量を制御した場合に、知覚バンディングが効果的に抑制されていることが判る。
【０１４５】
これにより、発光部ｃｈ１，ｃｈ４の発光量が中央の発光部ｃｈ２，ｃｈ３に対して９０％（露光量９０％）となるように各発光部ｃｈ１，ｃｈ２，ｃｈ３，ｃｈ４からの発光量を制御することにより、隣接走査を行なうマルチビーム走査装置１８での重畳露光により発生するバンディング現象を抑えることができ、品質の高い画像を形成することができる。
【０１４６】
次に、本発明の第８の実施の形態について図１８および図１９を参照して説明する。
【０１４７】
本実施の形態は、複写装置としてデジタル複写機への適用例を示す。
【０１４８】
図１８は、本実施の形態のデジタル複写機を示す縦断面図である。本実施の形態のデジタル複写機５０は、原稿の画像を読み取る画像読取装置としてのカラースキャナ装置５１と、スキャナ装置５１の上側に設けられた原稿給送装置５２と、カラースキャナ装置５１で読み取った原稿の画像データに基づいた画像形成を電子写真方式で行なう画像形成装置としてのカラーレーザプリンタ５３とを備えている。
【０１４９】
まず、カラースキャナ装置５１について説明する。カラースキャナ装置５１は、上面に原稿がセットされるコンタクトガラス５４を有するスキャナケース５５を備えている。
【０１５０】
スキャナケース５５の内部には、光を発光する露光ランプ５６およびミラー５７を搭載する第１走行体５８と、２枚のミラー５９，６０を搭載する第２走行体６１とが設けられている。露光ランプ５６およびミラー５７，５９，６０は、長手方向を主走査方向に平行にして配設されている。第１、第２走行体５８，６１は、コンタクトガラス５４に沿って副走査方向（図１中紙面左右方向）に往復動自在に設けられており、図示しないステッピングモータによって２：１の速度比で走行される。
【０１５１】
カラースキャナ装置５１は、露光ランプ５６を発光させるとともに第１、第２走行体５８，６１をコンタクトガラス５４に沿って往復動させ、露光ランプ５６の主走査方向の幅と露光ランプ５６が移動した距離とによって決定される範囲で原稿を露光走査する。
【０１５２】
原稿を露光走査した光は、ミラー５７，５９，６０によって導かれ、結像レンズ６２を介して、画像読み取り基板６３に搭載されたカラーイメージセンサ６４に結像される。
【０１５３】
画像読み取り基板６３は、原稿を介してカラーイメージセンサ６４に結像された露光光のアナログ画像データを光電変換してデジタル画像データを生成する。
【０１５４】
画像読み取り基板６３で生成したデジタル画像データは、例えば、シェーディング補正処理、ＭＴＦ補正処理、γ補正処理等の公知の各種画像処理が施されて以降の処理に供される。
【０１５５】
次に、原稿給送装置５２について説明する。原稿給送装置５２には、シート状の原稿がセットされる原稿テーブル６５が設けられており、原稿が複数枚ある場合には、原稿が原稿テーブル６５上に積み重ねた状態でセットされる。図１２中、原稿テーブル６５の左端位置に設けられた給紙口６６には、給紙口６６から原稿台上の読み取り位置を経由して排出口７３に至る原稿搬送経路６７が連通されている。
【０１５６】
特に図示しないが、原稿搬送経路６７と給紙口６６との連通位置には、原稿テーブル６５にセットされた原稿を原稿搬送経路６７中へ送り出す呼び出しコロや、呼び出しコロによって原稿搬送経路６７中へ送り出された原稿を１枚ずつに分離する分離機構等が設けられている。
【０１５７】
原稿搬送経路６７中には、複数の搬送ローラ７０に巻回されて、搬送ローラ７０によって回転されることで原稿を搬送する搬送ベルト７１が設けられている。搬送ベルト７１は、コンタクトガラス５４を全体に亘って覆うようにして設けられており、原稿を搬送するとともにコンタクトガラス５４上の原稿をコンタクトガラス５４に押し当てて原稿がコンタクトガラス５４から浮き上がることを防止する機能を果たしている。呼び出しコロ、分離機構および複数の搬送ローラは、図示しないモータによって回転駆動される。
【０１５８】
加えて、原稿搬送経路６７には、コンタクトガラス５４上に搬送された原稿を読み取り位置で停止させるための図示しないストップ爪等が設けられている。ストップ爪は、露光走査が終了すると、原稿搬送経路６７を開放するように変位し、これにより原稿の排出が可能となる。排出口７３から排出された原稿は、排出台７４に積層される。
【０１５９】
原稿給送装置５２は、呼び出しコロによって原稿搬送経路６７中に送り込み、分離機構によって１枚ずつに分離した１枚の原稿のみを、搬送ベルト７１によって搬送してコンタクトガラス５４上に送り出す。そして、該原稿が読み取り位置に到達した時点で搬送ローラ７０の回転を停止させる。これにより、原稿がコンタクトガラス５４上に位置付けられる。なお、原稿の搬送方向の寸法は、例えば、原稿搬送経路中に図示しないセンサを設け、このセンサによる原稿検出のタイミング等から取得することができ、この検出タイミングに基づいてモータを駆動することにより原稿をコンタクトガラス５４上に正確に位置付けることができる。
【０１６０】
原稿給送装置５２は、原稿がコンタクトガラス５４上に位置付けられている状態で、カラースキャナ装置５１による上述した原稿の露光走査が終了すると、搬送ローラ７０の駆動を再開し、コンタクトガラス５４上の原稿を排出台７４へ搬送する。搬送ベルト７１はコンタクトガラス５４を全体に亘って覆うように設けられているため、呼び出しコロおよび分離機構の駆動タイミングを調整することで、コンタクトガラス５４上の原稿を排出台７４へ搬送すると同時に、次の原稿を給紙することができる。
【０１６１】
次に、カラーレーザプリンタ５３について説明する。カラーレーザプリンタ５３内には、給紙部７５から、縦搬送部７６、プリンタエンジン７７、定着部７８等を経由して排紙トレイ７９へ至る記録媒体案内路８０が形成されている。
【０１６２】
給紙部７５は、記録媒体３を積層保持する第１，第２給紙トレイ８４，８５と、各給紙トレイ８４，８５に設けられて積層された記録媒体３を１枚ずつに分離して縦搬送部に給送する給紙機構８７，８８とを備えている。給紙機構８７，８８は、それぞれピックアップローラ９０や図示しないリバースローラ等によって構成されており、各給紙トレイ８４，８５，８６に積層保持された記録媒体３を１枚ずつに分離して記録媒体案内路へ送り出す。ピックアップローラ９０やリバースローラは、図示しないモータによって回転駆動される。
【０１６３】
縦搬送部７６には、給紙トレイ８４，８５給紙された記録媒体３を、それぞれプリンタエンジン７７へ搬送するための複数の搬送ローラ９２が設けられている。
【０１６４】
プリンタエンジン７７は、複数のローラ９３に巻回された被転写体としての中間転写ベルト９４に対して、Ｙ，Ｍ，Ｃ，Ｋの各色のカラー画像を形成するタンデム方式のプリンタエンジンであり、Ｙ，Ｍ，Ｃ，Ｋの色毎に設けられた感光体９５（９５Ｙ，９５Ｍ，９５Ｃ，９５Ｋ）を備えている。各感光体９５の周囲には、各感光体９５表面を帯電させる帯電装置としての帯電ローラ９６、所定の色画像データに基づいて対応する感光体９５をそれぞれ光走査するマルチビーム走査装置９７、マルチビーム走査装置９７によって感光体９５表面に書き込んだ静電潜像に所定色のトナーをそれぞれ付着させる現像装置９８、各感光体９５表面に形成された各色のトナー像を中間転写ベルト９４にそれぞれ転写させる中間転写装置９９、中間転写ベルト９４上に形成された単色ないし複数色のトナー像を記録媒体案内路８０中を搬送される記録媒体３に転写させる第２の転写装置としての転写部１００、各感光体９５に残存するトナーを除去するクリーナー１０１、および、中間転写ベルト９４上に残存するトナーを除去する図示しないクリーナー等を有している。
【０１６５】
本実施の形態のマルチビーム走査装置９７は、各感光体９５に対して、それぞれ４ラインずつ光走査することによって、各色に対応した静電潜像を書き込む。マルチビーム走査装置９７は、電圧値Ｖ_Ｌの静電潜像を感光体９５表面に書き込む。
【０１６６】
マルチビーム走査装置９７は、第１の実施の形態のマルチビーム走査装置１８と同様の構成であるため、図示および説明を省略するが、各感光体９５に対応する４ｃｈの半導体レーザアレイを具備している。なお、各半導体レーザアレイから発光されるビームを対応する感光体９５に導く光学部品は、半導体レーザアレイ毎に個別に設けられていてもよいし、共通化されていてもよく、あるいは、一部が共通化されていてもよい。
【０１６７】
本実施の形態の中間転写装置９９は、図１９に示すように、中間転写ベルト９４を介して各感光体９５に対向されて感光体９５との間に転写バイアスを印加させる第１の転写装置としての中間転写バイアスローラ９９ａと、各中間転写バイアスローラ９９ａよりも中間転写ベルト９４の回転方向上流側に配設された印加電圧部材９９ｂとを備えている。印加電圧部材９９ｂは、感光体９５の帯電極性と同極性で、かつ、感光体９５表面に書き込まれる静電潜像の電圧値Ｖ_Ｌよりも大きい絶対値を有する電圧を、中間転写ベルト９４に対して印加する。
【０１６８】
感光体９５表面に形成されたトナー像の中間転写ベルト９４への転写に際しては、上述した電圧を印加電圧部材９９ｂによって中間転写ベルト９４に対して印加した後、中間転写バイアスローラ９９ａによって転写バイアスを印加する。これにより、トナー像が中間転写バイアスローラ９９ａによる転写領域に入る前に中間転写ベルト９４に転写されること（プレ転写）を防止して、各感光体９５から中間転写ベルト９４へのトナー転写に際して、チリ等によってトナー像が乱れることを防止し、経時変動や現像剤のバラツキに依らず安定した画像を形成することができる。
【０１６９】
中間転写ベルト９４上には、各感光体９５で形成されたトナー像が転写されることでフルカラー画像が形成される。中間転写ベルト９４上に形成されたフルカラー画像は、転写部１００によって記録媒体に転写される。
【０１７０】
本実施の形態のデジタル複写機５０によれば、副走査方向の重畳長さを最小画素以下とすることにより感光体９５表面に形成される静電潜像にバンディングが発生することを防止することで、感光体９５表面に形成されるトナー像の品質がバンディングに起因して低下すること防止するとともに、各感光体９５表面に形成されたトナー像を中間転写ベルト９４上に良好に転写することができ、カラースキャナ装置５１が読み取った原稿画像に忠実な画像を複写形成することができる。
【０１７１】
【発明の効果】
請求項１記載の発明の画像形成装置によれば、被走査面上に形成される走査線の副走査方向の重畳長さを最小画素以下とすることにより、高解像度での書き込みを行なう場合にも解像度に左右されることなく、重畳露光によるバンディングの発生を防止することができ、高解像度での書き込みを行なう場合にも解像度に左右されることなく、重畳露光によるバンディングの発生を防止して、元画像に忠実で高品質な画像を形成することができる。
【０１７２】
請求項２記載の発明によれば、請求項１記載の画像形成装置において、２１．１７μｍ以下が最小画素以下となる解像度が１２００ｄｐｉである場合には、副走査方向の重畳長さを２１μｍ以下とすることにより重畳露光によるバンディングの発生を防止して、元画像に忠実で高品質な画像を形成することができる。
【０１７３】
請求項３記載の発明によれば、請求項１または２記載の画像形成装置において、ビームスポットの副走査方向における径を４２μｍ以下とすることにより、ビームスポット径の小型化に伴って減少するビームスポットの副走査方向における重畳長さを最小画素以下とすることができ、重畳露光によるバンディングの発生を防止して、元画像に忠実で高品質な画像を形成することができる。
【０１７４】
請求項４記載の発明によれば、請求項１、２または３記載の画像形成装置において、感光体の量子効率ηを、地汚れを防止して良好な画像形成を可能とする０．４＜η＜１の範囲内に設定することにより、バンディングの発生および地汚れを防止して元画像に忠実な高品質で良好な画像を形成することができる。
【０１７５】
請求項５記載の発明によれば、請求項１ないし４のいずれか一に記載の画像形成装置において、入出力特性の直線回帰時の相関係数Ｒ^２値を０．９７以上とすることにより、被走査面上を１回で同時に光走査した場合のライン幅と、２回で順次光走査した場合のライン幅との差によって発生するバンディングの影響を受け易い状況下でも、バンディングの発生を防止することができる。
【０１７６】
請求項６記載の発明によれば、請求項１ないし５のいずれか一に記載の画像形成装置において、臨界電界強度以上の領域におけるトナー付着量Ｍ／Ａを０．２≦Ｍ／Ａ≦０．８ｍｇ／ｃｍ^２とすることにより、感光体表面に一層分のトナーを付着させる低Ｍ／Ａ現像を行なうことにより、被走査面上に形成される走査線の副走査方向の重畳長さを最小画素の２倍以下とすることができ、低Ｍ／Ａ現像を行なわない画像形成装置と比較して、解像度に左右されずに、重畳露光によるバンディングの発生を防止することができる重畳長さの範囲を広げることができる。
【０１７７】
請求項７記載の発明によれば、請求項６記載の画像形成装置において、ビームスポットの副走査方向における径を６３μｍ以下とすることにより、ビームスポット径の小型化に伴って減少するビームスポットの副走査方向における重畳長さを最小画素以下とすることができ、低Ｍ／Ａ現像を行なわない画像形成装置と比較して、解像度に左右されずに、重畳露光によるバンディングの発生を防止することができるビームスポットの副走査方向における径の範囲を広げることができる。
【０１７８】
請求項８記載の発明の画像形成装置によれば、被走査面上において副走査方向の両端部を光走査する光量を、副走査方向の両端部以外を光走査する光量よりも低くすることにより、順次露光時のライン幅と同時露光時のライン幅との差を低減させることができ、重畳露光によるバンディングの発生を防止して、元画像に忠実で高品質な画像を形成することができる。
【０１７９】
請求項９記載の発明によれば、請求項８記載の画像形成装置において、被走査面上において副走査方向の両端部を光走査する光量を、副走査方向の両端部以外を光走査する光量に対して９０％とすることにより、順次露光時のライン幅と同時露光時のライン幅との差を確実に低減させることができ、重畳露光によるバンディングの発生を防止することができ、元画像に忠実で高品質な画像を形成することができる。
【０１８０】
請求項１０記載の発明によれば、請求項１ないし９のいずれか一に記載の画像形成装置において、第１の転写装置による被転写体へのトナーの転写に先立って、感光体の帯電極性と同極性であり、該感光体表面に形成される静電潜像の電位Ｖ_Ｌよりも絶対値が大きい電圧を印加することにより、第１の転写装置によるトナーの転写前に感光体上のトナーが被転写体にプレ転写されたり感光体上のトナーに対して放電されたりすることを防止することができるので、重畳露光によるバンディングの発生を防止するとともに、画像チリを防止し、経時変動や現像剤の性質等のバラツキに依らず安定した画像を形成することができる。
【０１８１】
請求項１１記載の発明の複写装置によれば、請求項１ないし１０のいずれか一に記載の発明の作用を有し、品質の高い画像を形成することができるので、画像読取装置が読み取った原稿画像に忠実な画像を形成することができる。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態のレーザプリンタを示す縦断面図である。
【図２】マルチビーム走査装置を示す斜視図である。
【図３】静電潜像の副走査方向におけるライン径と電界強度との相関を示すグラフである。
【図４】同時光走査によって主走査方向に沿って２ドットライン静電潜像を形成した場合のキャリア発生量と、順次光走査によって主走査方向に沿って２ドットライン静電潜像を形成した場合のキャリア発生量との違いを示すグラフである。
【図５】同時光走査によって静電潜像を形成した場合の電界強度分布と、順次光走査によって同じ静電潜像を形成した場合の電界強度分布とを示すグラフである。
【図６】書込密度１２００ｄｐｉのマルチビーム走査方式で、同時露光と順次露光との２方式により２ドット主走査ラインを露光することによって静電潜像を形成した場合の同時露光によるライン断面積と順次露光によるライン断面積との差（ライン断面積差）と知覚バンディングとの関係を示すグラフである。
【図７】副走査方向のビーム重畳長さとライン断面積差との関係を示すグラフである。
【図８】本発明の第２の実施の形態のレーザプリンタにおいて、ビームスポットの副走査方向の径と、静電潜像ラインの副走査方向での重畳長さとの関係を示すグラフである。
【図９】ビームスポットの副走査方向における径とライン幅との関係を示すグラフである。
【図１０】本発明の第３の実施の形態のレーザプリンタの感光体における光減衰特性を示すグラフである。
【図１１】本発明の第４の実施の形態のレーザプリンタにおける入出力特性の一例を示すグラフである。
【図１２】本発明の第５の実施の形態のレーザプリンタで副走査方向における静電潜像ライン幅と知覚バンディングとの関係を示すグラフである。
【図１３】同時露光時の静電ライン幅と順次露光時のライン幅との差（ライン幅差）と、副走査方向の重畳長さとの関係を示すグラフである。
【図１４】本発明の第６の実施の形態のレーザプリンタにおけるビームスポット径と副走査方向における重畳長さとの関係を示すグラフである。
【図１５】本発明の第７の実施の形態の本実施の形態のレーザプリンタで、順次露光部分の露光量を１００％として露光を行なった場合の露光量分布と、順次露光部分の露光量を９０％として露光を行なった場合の露光量分布とを示すグラフである。
【図１６】重畳露光量と知覚バンディングとの関係を示すグラフである。
【図１７】順次露光時の露光量の変化による知覚バンディングの変化の関係を示すグラフである。
【図１８】本発明の第８の実施の形態のデジタル複写機を示す縦断面図である。
【図１９】中間転写装置を示す拡大図である。
【符号の説明】
１　　　画像形成装置
３　　　被転写体
１３　　　感光体
１４　　　帯電装置
１５　　　現像装置
１６　　　転写装置
１８　　　マルチビーム走査装置
５０　　　複写装置
５１　　　画像読取装置
５３　　　画像形成装置
９４　　　被転写体
９５　　　感光体
９６　　　帯電装置
９７　　　マルチビーム走査装置
９８　　　現像装置
９９ａ　　第１の転写装置
９９ｂ　　電圧印加部材
１００　　第２の転写装置
ｃｈ１，ｃｈ２，ｃｈ３，ｃｈ４　　光源[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus and a copying apparatus.
[0002]
[Prior art]
For example, an optical scanning device which is used in an image forming apparatus such as a digital copying machine or an optical printer and optically scans a surface to be scanned such as a photoreceptor surface is widely known. Optical scanning by an optical scanning device is performed by forming a light spot on a surface to be scanned and scanning the surface to be scanned in the main scanning direction with the light spot.
[0003]
In such an optical scanning device, as a method for increasing the speed of optical scanning, a “multi-beam method” in which optical scanning is simultaneously performed on a surface to be scanned by a plurality of scanning lines (lines) has attracted attention. As a “multi-beam scanning device”, a “multi-beam scanning device” using a semiconductor laser array having a plurality of light-emitting portions monolithically arranged in one row is being realized. In a multi-beam scanning device, a plurality of light spots are formed on a surface to be scanned so as to be arranged in the sub-scanning direction, and light scanning is performed on the surface to be scanned in the main scanning direction by these light spots.
[0004]
In a multi-beam scanning device, by using a semiconductor laser array, the interval between the light emitting units can be kept constant and the pitch of the light emitting units can be reduced, and a plurality of light emitting units can be arranged on the optical path from the light source to the surface to be scanned. An optical system for guiding a plurality of beams emitted from the light source can be shared. Thereby, stability against mechanical fluctuations can be improved.
[0005]
In a multi-beam scanning device, optical scanning is performed using a semiconductor laser array having a plurality of light-emitting portions. Therefore, the pitch of scanning lines formed by optical scanning of a surface to be scanned by a light spot increases the writing density. It narrows with it. As the scanning line pitch decreases, the diameter of the light spot in the sub-scanning direction also needs to be reduced.
[0006]
The multi-beam scanning method includes an “adjacent scanning” method in which adjacent scanning lines are optically scanned at the same time and an “interlaced scanning” method in which optical scanning is performed at intervals of one or more scanning lines. In the interlaced scanning system, there is a problem that the so-called scanning line pitch unevenness is more likely to occur, as compared with the adjacent scanning system, in which the interval between scanning lines that are simultaneously optically scanned varies with the optical scanning.
[0007]
As described above, since the pitch of the scanning line becomes smaller as the density becomes higher during optical scanning, the scanning line pitch becomes smaller in the optical scanning device employing the interlaced scanning method as the density becomes higher during optical scanning. In contrast, the scanning line pitch unevenness becomes relatively large. Such an influence of the scanning line pitch unevenness occurs remarkably when the writing density is higher than 600 dpi, and the quality of an image formed on the surface to be scanned is likely to deteriorate with the higher writing density. .
[0008]
For this reason, the “adjacent scanning” method is suitable for forming a high-density image in the multi-beam scanning device.
[0009]
[Problems to be solved by the invention]
By the way, in a multi-beam scanning apparatus that performs optical scanning by the adjacent scanning method, if the diameter of the light spot in the sub-scanning direction is reduced along with the increase in the density of the formed image, an abnormal image due to the LDA (Laser Diode Array) pitch occurs. Sometimes.
[0010]
For example, in a multi-beam scanning apparatus using a 4ch-LDA in which four LDs having a stationary beam spot diameter of 55 ± 6 μm in the main scanning direction and a stationary beam spot diameter of 70 ± 7 μm in the sub-scanning direction have a resolution of 1200 dpi. When a 170 lpi halftone image is formed, banding at a pitch of 0.42 mm, which is the least common multiple of a pitch of 0.1056 mm of a line (scanning line) optically scanned in the sub-scanning direction and a pitch of 0.0846 mm of 4ch-LDA, is generated. appear.
[0011]
There is a concern that the effect of this banding will increase as the resolution increases.
[0012]
An object of the present invention is to prevent occurrence of banding due to superimposed exposure and to form a high-quality image faithful to an original image regardless of resolution even when writing at high resolution. .
[0013]
[Means for Solving the Problems]
The image forming apparatus according to the first aspect of the present invention emits light from a plurality of light sources, with a photosensitive member, a charging device for charging the surface of the photosensitive member, and the surface of the photosensitive member charged by the charging device as a surface to be scanned. A multi-beam scanning device that simultaneously optically scans the surface to be scanned with a plurality of light beams by deflecting and scanning light; and a developing device that supplies toner to the photoconductor surface optically scanned by the multi-beam scanning device. And a transfer device for transferring the toner supplied to the surface of the photoconductor by the developing device to a transfer target, wherein a scan formed on the surface to be scanned by simultaneous light scanning by the multi-beam scanning device is provided. The overlap length of the line in the sub-scanning direction is set to be equal to or less than the minimum pixel.
[0014]
Therefore, by setting the superimposed length of the scanning line formed on the surface to be scanned in the sub-scanning direction to be the minimum pixel or less, even when writing at a high resolution, the superimposed exposure can be performed without being affected by the resolution. Banding can be prevented from occurring.
[0015]
According to a second aspect of the present invention, in the image forming apparatus of the first aspect, when the resolution is 1200 dpi, the superimposed length of the scanning lines in the sub-scanning direction is set to 21 μm or less.
[0016]
Therefore, when the resolution is 1200 dpi, the minimum pixel is 21.17 μm or less. Therefore, by setting the superimposition length in the sub-scanning direction to 21 μm or less, banding due to superimposition exposure can be prevented.
[0017]
According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the resolution is 1200 dpi, and a light beam that optically scans the surface to be scanned simultaneously is a beam spot formed on the surface to be scanned. Diameter is 1 / e of maximum beam intensity ² In this case, the diameter of the beam spot in the sub-scanning direction is set to 42 μm or less.
[0018]
Therefore, by setting the diameter of the beam spot in the sub-scanning direction to 42 μm or less, the superimposed length in the sub-scanning direction of the beam spot, which decreases as the beam spot diameter decreases, can be made the minimum pixel or less.
[0019]
According to a fourth aspect of the present invention, in the image forming apparatus of the first, second, or third aspect, the quantum efficiency η of the photoconductor is set to 0.4 <η <1.
[0020]
Therefore, by setting the quantum efficiency η of the photoreceptor in the range of 0.4 <η <1, which prevents the background contamination and enables good image formation, the occurrence of banding and the background contamination can be prevented. It is possible to form a high-quality and good image faithful to the original image.
[0021]
According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, the correlation coefficient R at the time of linear regression of the input / output characteristics is provided. ² The value is set to 0.97 or more.
[0022]
Therefore, the correlation coefficient R at the time of linear regression of the input / output characteristics ² By setting the value to 0.97 or more, the influence of banding caused by the difference between the line width when optical scanning is simultaneously performed on the surface to be scanned once and the line width when optical scanning is sequentially performed twice is performed. Banding can be prevented from occurring even under conditions that are easily affected.
[0023]
According to a sixth aspect of the present invention, in the image forming apparatus according to any one of the first to fifth aspects, the developing device applies a toner having a colorant content of 5 to 15 w% to the photoconductor. And the toner adhesion amount M / A in a region equal to or higher than the critical electric field intensity, which is the electric field intensity at which toner adheres according to the electric field intensity distribution formed on the photoreceptor surface by optical scanning of the multi-beam scanning device, .2 ≦ M / A ≦ 0.8 mg / cm ² And
[0024]
Therefore, the toner adhesion amount M / A in the region equal to or higher than the critical electric field intensity is set to 0.2 ≦ M / A ≦ 0.8 mg / cm. ² By doing so, the effect of the invention described in any one of claims 1 to 5 can be obtained in an image forming apparatus that performs low M / A development in which one layer of toner adheres to the surface of the photoconductor.
[0025]
According to a seventh aspect of the present invention, in the image forming apparatus according to the sixth aspect, when the resolution is 1200 dpi, a light beam that simultaneously optically scans the surface to be scanned is a sub-beam of a beam spot formed on the surface to be scanned. The diameter in the scanning direction is 63 μm or less.
[0026]
Therefore, by setting the diameter of the beam spot in the sub-scanning direction to 63 μm or less, it is possible to reduce the overlap length in the sub-scanning direction of the beam spot, which decreases as the beam spot diameter is reduced, to the minimum pixel or less.
[0027]
The image forming apparatus according to claim 8, wherein a plurality of light sources emit light with a photosensitive member, a charging device for charging the surface of the photosensitive member, and the surface of the photosensitive member charged by the charging device as a surface to be scanned. A multi-beam scanning device that simultaneously optically scans the surface to be scanned with a plurality of light beams by deflecting and scanning light; and a developing device that supplies toner to the photoconductor surface optically scanned by the multi-beam scanning device. And a transfer device for transferring the toner supplied to the surface of the photoreceptor by the developing device to a transfer-receiving member. In the simultaneous light scanning by the multi-beam scanning device, a sub-scan is performed on the surface to be scanned. The light quantity for optical scanning at both ends in the direction is made lower than the light quantity for optical scanning at parts other than both ends in the sub-scanning direction.
[0028]
Therefore, by setting the light amount for optically scanning both ends in the sub-scanning direction on the surface to be scanned lower than the light amount for optically scanning other than the both ends in the sub-scanning direction, the line width at the time of sequential exposure and the light amount at the time of simultaneous exposure are reduced. The difference from the line width can be reduced.
[0029]
According to a ninth aspect of the present invention, in the image forming apparatus according to the eighth aspect, at the time of simultaneous light scanning by the multi-beam scanning device, the light amount for optically scanning both ends in the sub-scanning direction on the surface to be scanned is determined by sub-scanning. 90% of the light amount for optical scanning other than both ends in the direction.
[0030]
Therefore, by setting the light quantity for optical scanning at both ends in the sub-scanning direction on the surface to be scanned to 90% of the light quantity for optical scanning at parts other than the both end parts in the sub-scanning direction, the light width at the same time as the line width at the time of sequential exposure is obtained The difference from the line width at the time of exposure can be reliably reduced.
[0031]
According to a tenth aspect of the present invention, in the image forming apparatus according to any one of the first to ninth aspects, the multi-beam scanning device includes a potential V on the surface of the photoconductor. _L A first transfer device for forming an electrostatic latent image of the toner image on the photosensitive member, and a transfer device configured to transfer the toner supplied to the surface of the photosensitive member by the developing device in contact with the photosensitive member to transfer the toner to the transfer member; A second transfer device that transfers the toner transferred on the transfer member to a recording medium; and a charging polarity of the photosensitive member that is provided upstream of the transfer position of the first transfer device in the rotation direction of the photosensitive member. Potential V with the same polarity _L A voltage applying member for applying a voltage having a larger absolute value than the voltage applying member.
[0032]
Therefore, prior to the transfer of the toner to the transfer target by the first transfer device, the potential V of the electrostatic latent image formed on the surface of the photoconductor is the same as the polarity of the charge of the photoconductor. _L By applying a voltage having an absolute value larger than that of the toner by the voltage applying member, the toner on the photosensitive member is pre-transferred to the transfer receiving member before the toner is transferred by the first transfer device, or the toner on the photosensitive member is Discharge can be prevented.
[0033]
The copying apparatus according to the eleventh aspect of the invention is an image reading apparatus for reading an original image, and an image based on the original image read by the image reading apparatus is formed on a transfer receiving body. And an image forming apparatus as described above.
[0034]
Therefore, it has the effect of the invention according to any one of claims 1 to 10 and can form a high quality image.
[0035]
BEST MODE FOR CARRYING OUT THE INVENTION
A first embodiment of the present invention will be described with reference to FIGS. This embodiment shows an example of application to a laser printer as an image forming apparatus.
[0036]
FIG. 1 is a longitudinal sectional view schematically showing a laser printer according to the present embodiment. The housing 2 of the laser printer 1 is provided with a paper feed cassette 4 for storing sheet-shaped recording media 3 in a stacked state. The recording medium stored in the paper feed cassette may be paper such as plain paper, or may be an OHP sheet or the like.
[0037]
The paper cassette 4 is provided detachably with respect to the housing 2. As shown in FIG. 1, in the housing 2, the sheet feeding cassette 4 is attached to the housing 2, and the top sheet of the recording medium 3 stored in the sheet feeding cassette 4 is brought into contact with the housing 2. A roller 5 is provided.
[0038]
In the housing 2, a medium path 9 is provided from the paper feed cassette 4 to a paper output tray 8 via an electrophotographic printer engine 6 and a fixing device 7. The medium path 9 is provided with a transport roller (not shown) in addition to the registration roller pair 10 and the paper discharge roller pair 11. The recording medium 3 stored in the paper feed cassette 4 is fed to the medium path 9 by the paper feed roller 5, and is conveyed through the medium path by the pair of registration rollers 10, the pair of discharge rollers 11, the conveyance rollers, and the like. The medium path 9 is provided with a guide member 12 for regulating the path of the recording medium conveyed in the medium path 9.
[0039]
The printer engine 6 includes a photoconductive photoconductor 13 formed in a cylindrical shape and rotatable around an axis, a charging roller 14 as a charging device disposed around the photoconductor 13, and a developing device 15. And a transfer roller 16 as a transfer device, a cleaning device 17 and the like.
[0040]
Although not shown in the figure, since the technique is a known technique, the photoreceptor 13 includes a carrier generation layer (CGL) for generating carriers (charges) and carriers (charges) generated in the carrier generation layer (CGL). And a carrier transport layer (CTL: Charge Transport layer).
[0041]
The charging roller 14 is disposed with its outer peripheral surface in contact with the surface of the photoconductor 13, and charges the surface of the photoconductor 13 by applying a charging voltage to the photoconductor 13. In the present embodiment, the charging roller 14 that contacts the surface of the photoconductor 13 is used. However, the present invention is not limited to this. For example, a corona charger that charges the surface of the photoconductor 13 in a non-contact manner by a corona discharge method may be used. You may.
[0042]
The developing device 15 includes a case 15a that holds a developer containing a toner and a carrier, and a developing sleeve 15b that supplies the developer held in the case 15a to the surface of the photoconductor 13. The developing device 15 supplies toner to the surface of the photoconductor 13 by the operation of the developing sleeve 15b to visualize an electrostatic latent image written on the surface of the photoconductor 13 by optical scanning by a multi-beam scanning device 18 described later. I do.
[0043]
The transfer roller 16 transfers the toner adhering to the surface of the photoconductor 13 to the recording medium 3 conveyed in the medium path 9 by applying a transfer voltage between the transfer roller 16 and the photoconductor 13. In the present embodiment, the transfer medium is realized by the recording medium 3. The above-described registration roller pair 10 temporarily stops the recording medium 3 fed from the paper feed cassette 4, and the toner image formed on the surface of the photoconductor 13 is at a position where the photoconductor 13 and the transfer roller 16 face each other. The recording medium 3 is sent to the transfer position at the timing of moving to the transfer position. The transfer bias is applied by the transfer roller 16 to the recording medium 3 sent to the transfer position at the timing adjusted by the function of the registration roller pair 10 when passing through the transfer position. As a result, the toner image formed on the surface of the photoconductor 13 is electrostatically transferred to the recording medium 3.
[0044]
The fixing device 7 fixes the toner image on the recording medium 3 by heating and pressing the recording medium 3 onto which the toner image has been transferred. The recording medium 3 after the fixing of the toner image is discharged onto a discharge tray 8 via a pair of discharge rollers 11.
[0045]
The cleaning device 17 cleans the surface of the photoconductor 13 by removing residual toner, paper powder, and the like remaining on the surface of the photoconductor 13 after the transfer of the toner image.
[0046]
Further, in the present embodiment, the toner image formed on the surface of the photoconductor 13 is directly transferred to the recording medium 3 by using the recording medium 3 as a transfer object. However, the present invention is not limited to this. The toner image formed on the surface 13 may be transferred to the recording medium 3 via an intermediate transfer medium such as an intermediate transfer belt (not shown).
[0047]
In addition, the printer engine 6 includes a multi-beam scanning device 18 that writes an electrostatic latent image on the surface of the photoconductor 13 charged by the charging roller 14. Here, FIG. 2 is a perspective view showing the multi-beam scanning device 18. The multi-beam scanning device 18 includes a semiconductor laser array 19 in which four light emitting units ch1, ch2, ch3, and ch4 as light sources are arranged in a line at equal intervals. The four light emitting units ch1, ch2, ch3, and ch4 in the semiconductor laser array 19 are arranged along the sub-scanning direction with the inclination angle θ with respect to the main scanning direction being 0.
[0048]
In the present embodiment, as shown in FIG. 2, the inclination angles θ of the four light emitting units ch1, ch2, ch3, and ch4 with respect to the main scanning direction are set to 0, and the semiconductor laser arrays are arranged along the sub-scanning direction. Although the number is set to 19, the present invention is not limited to this, and the four light emitting units ch1, ch2, ch3, and ch4 may be inclined with respect to the sub-scanning direction (tilt angle: θ> 0).
[0049]
The four beams emitted from the four light emitting units ch1, ch2, ch3, and ch4 are incident on the coupling lens 20 as divergent light beams having an elliptical far-field pattern whose main axis is the main scanning direction. Coupled. The coupling lens 20 is common to the four beams.
[0050]
The form of each beam to be coupled by the coupling lens 20 may be a weakly divergent light beam, a weakly convergent light beam, or a parallel light beam, depending on the optical characteristics of the subsequent optical system.
[0051]
The four beams transmitted through the coupling lens 20 are beam-shaped by passing through the aperture 21a of the aperture member 21, and are converged in the sub-scanning direction by a cylindrical lens 22, which is a common imaging optical system. An image is formed in the vicinity of the deflecting and reflecting surface 23a of the rotating polygon mirror 23 as a container. At this time, the four beams imaged in the vicinity of the deflecting / reflecting surface 23a are each formed into a long line image in the main scanning direction, and are imaged separately from each other in the sub scanning direction.
[0052]
The rotary polygon mirror 23 is driven to rotate at a constant angular velocity by a motor (not shown). As a result, the four beams imaged in the vicinity of the deflecting / reflecting surface 23a are deflected at a uniform angular velocity by the deflecting / reflecting surface 23a, pass through the two lenses 24 and 25 forming the scanning image forming optical system, and are bent. The light enters the mirror 26.
[0053]
Each beam incident on the bending mirror 26 is bent in the optical path by the bending mirror 26 and is condensed on the surface of the photoconductor 13 as a surface to be scanned as four light spots separated in the sub-scanning direction. The four light spots condensed on the surface of the photoconductor 13 optically scan the surface of the photoconductor 13 in the main scanning direction. As a result, four scanning lines are simultaneously scanned adjacently on the surface of the photoconductor 13. According to the multi-beam scanning device 18 of the present embodiment, at the time of optical scanning, adjacent scanning lines are simultaneously optically scanned by beams emitted from the four light emitting units ch1, ch2, ch3, and ch4 (adjacent scanning method). .
[0054]
One of the four beams formed near the deflecting / reflecting surface 23a is incident on the mirror 27 prior to optical scanning, and is condensed on the light receiving element 29 via the lens 28. When writing the electrostatic latent image, the writing start timing of the four beams is determined based on the output from the light receiving element 29.
[0055]
The multi-beam scanning device 18 forms an electrostatic latent image (negative electrostatic latent image) on the surface of the photoconductor by exposing a portion to be an image.
[0056]
As described above, when the multi-beam scanning device 18 irradiates (exposes) light energy on the charged surface of the photoconductor 13 to an image forming portion, the carrier (charge) is applied to the carrier generation layer inside the photoconductor 13. Is generated, and the generated carrier moves through the carrier moving layer in the photoconductor 13 to neutralize the charge on the surface of the photoconductor 13. As a result, an electrostatic latent image (negative electrostatic latent image) is formed on the surface of the photoconductor 13, and an electric field intensity distribution is formed on the surface of the photoconductor 13 by the formed electrostatic latent image. The toner supplied by the developing device 15 adheres to an area having a certain threshold electric field strength or more according to such an electric field intensity distribution. As a result, a toner image is formed on the surface of the photoconductor 13.
[0057]
At this time, the electrostatic latent image charge amount and the electric field intensity distribution on the surface of the photoconductor 13 are determined by the carrier generation amount, Coulomb repulsion between carriers, recombination of carriers, and carrier mobility.
[0058]
By the way, when the above-described adjacent scanning method is adopted in the optical scanning by the multi-beam, an abnormal image may be generated at the pitch (LDA pitch) of the light emitting units ch1, ch2, ch3, and ch4 in the semiconductor laser array 19. For example, in a laser printer using a 4ch-LDA in which four LDs having a stationary beam spot diameter of 55 ± 6 μm in the main scanning direction and a stationary beam spot diameter of 70 ± 7 μm in the sub-scanning direction have a resolution of 1200 dpi and a resolution of 170 lpi. When forming a halftone image of (1), banding is performed at a pitch of 0.42 mm, which is a least common multiple of a scanning line pitch (line pitch) in the sub-scanning direction (line pitch) of 0.1056 mm and a 4-ch LDA pitch of 0.0846 mm. appear.
[0059]
Such banding is such that, when lines adjacent in the sub-scanning direction are formed by multi-beam scanning, the line width when optical scanning is performed in one scan by simultaneous exposure is larger than the line width when sequentially exposed in two scans. This occurs because lines formed by thin and simultaneous exposure are periodically present.
[0060]
The following is a calculation of the electrostatic latent image charge density distribution and the electric field intensity distribution by calculation, and based on the calculated electrostatic latent image charge density distribution and the electric field intensity distribution, a line in the case of performing optical scanning in one scan by simultaneous exposure. A study of the difference between the width and the line width when sequentially exposing in two scans will be described.
[0061]
The electrostatic latent image charge density distribution and the electric field intensity distribution used for studying the difference in line width can be calculated by a calculation method having the following three steps.
[0062]
1. Calculation of electrostatic latent image charge distribution
First, an electrostatic latent image charge distribution on the photoconductor 13 after exposure is calculated. The movement of carriers (charges) inside the photoconductor 13 is affected by Coulomb repulsion between carriers, recombination between carriers, and mobility of carriers inside the photoconductor. For this reason, in the present embodiment, as the electrostatic latent image forming simulation taking all of these effects into consideration, (1) calculation of the exposure amount by the Gaussian laser beam, and (2) calculation of the generation of the charge carrier and its transport process. The analysis was performed using a physical model consisting of
[0063]
(1) Exposure calculation with Gaussian laser beam
Here, the exposure amount distribution of the single stationary beam was approximated by the equation (1), and the exposure amount was calculated by integrating in the X direction by the moving distance (Vx * lighting time) during the lighting time.
[0064]
(Equation 1)

[0065]
(2) Charge carrier generation and transport process
The process of charge carrier generation and transport in the OPC (organic photoreceptor) as the photoreceptor 13 is represented by the following formulas (2), (3), and (4). Ruled.
[0066]
(Equation 2)

[0067]
Here, n: number density of carriers, μ: mobility of carriers, E: electric field strength, Δ: amount of carriers generated per unit time, r: recombination coefficient per unit time of carriers, ε: dielectric constant, e : Elementary charge, p: positive carrier, n: positive / negative carrier
[0068]
Here, assuming that charge carriers are uniformly generated in the layer because the carrier generation layer is thin, the amount of carrier generation Γ and the incident light intensity F, the quantum efficiency η, and the thickness d of the carrier generation layer are determined. The relationship is represented by the following equation (5).
[0069]
[Equation 3]

[0070]
Here, β: light absorption efficiency in the carrier generation layer, η: quantum efficiency, hυ: energy per photon of the laser beam
[0071]
The quantum efficiency η depends on the electric field, η = αE ⁿ Is represented by
[0072]
The second item on the right side of the above equations (2) and (3) is a carrier introduced to explain that the amount of generated carriers decreases experimentally when positive and negative carriers coexist in the same vicinity. It is a recombination term.
[0073]
Of the above physical quantities, the light absorption coefficient β and the recombination coefficient R are calculated by performing an experimental fitting from the surface potential at the time of solid black exposure.
[0074]
Then, assuming that the surface of the photoconductor 13 before exposure is uniformly charged, the amount of charge on the surface of the photoconductor 13 is calculated, and then the above equations (2), (3), and (4) are calculated. As a result, the charge density distribution on the surface of the photoconductor 13 after the exposure was calculated. Hereinafter, the charge density distribution on the surface of the photoconductor 13 after the exposure is referred to as “charge distribution of the electrostatic latent image on the photoconductor”.
[0075]
2. Development electric field strength distribution calculation
Next, a development electric field intensity distribution is calculated from the electrostatic latent image charge distribution on the photoconductor obtained by the above-described calculation and the development conditions. Here, assuming that the developing sleeve 15b and the photoreceptor 13 are approximated by a parallel plate and that the developer (carrier and toner) is uniformly filled between them, the layer formed by the developer is referred to as an average dielectric constant. It can be regarded as a uniform dielectric layer having ε. The average dielectric constant ε in this dielectric layer is the average relative dielectric constant ε ′ and the vacuum dielectric constant ε. ₀ (See equation (6)). At this time, the average relative permittivity ε ′ is obtained by the calculation of the following equation (7).
[0076]
(Equation 4)

[0077]
Where ε ₁ : Relative permittivity of carrier, a: ratio of volume occupied by carrier in development nip
[0078]
The potential distribution on the surface of the photoconductor 13 is obtained by solving the Poisson equation with the surface of the development sleeve 15b as a development bias potential, the lower surface of the photoconductor 13 as a constant boundary condition at 0 V, and the left and right boundaries as periodic boundary conditions. Then, based on the obtained potential distribution on the surface of the photoconductor 13, the following formula (8) is operated to obtain a development electric field intensity distribution on the surface of the photoconductor 13.
[0079]
(Equation 5)

[0080]
3. Calculation of electrostatic latent image dot diameter and electrostatic latent image cross-sectional area value
The toner image on the surface of the photoconductor 13 is formed by the toner adhering to a region above a certain threshold electric field intensity according to the development electric field intensity distribution on the surface of the photoconductor 13. In the present embodiment, this "certain threshold electric field intensity" is defined as a critical developing electric field intensity, and is set to a value determined by the development process and the conditions of the developer.
[0081]
Here, FIG. 3 is a graph showing the correlation between the line diameter in the sub-scanning direction of the electrostatic latent image and the electric field strength. As described above, the toner adheres to the region of the critical developing electric field intensity or higher in the electric field intensity distribution at the time of development. Therefore, if the line Eth in FIG. Adhere to the region having In the present embodiment, since the electric field strength with respect to the line diameter in the sub-scanning direction of the electrostatic latent image is taken into consideration, the toner adheres to a region having an electric field strength equal to or more than the hatched line Eth in FIG. In FIG. 3, the length in the sub-scanning direction of a region in the electric field intensity distribution during development that is equal to or higher than the critical developing electric field intensity is defined as the line diameter of the electrostatic latent image in the sub-scanning direction. The toner adhesion amount corresponds to the cross-sectional area of a line in the sub-scanning direction of the electrostatic latent image.
[0082]
In the present embodiment, the line diameter of the electrostatic latent image in the sub-scanning direction is used. However, the present invention is not limited to this. Consider.
[0083]
For this reason, if the critical developing electric field strength is known, the diameter of the area on the photoconductor 13 where the toner is attached, that is, the electrostatic latent image dot diameter (line diameter if a line), and electrostatic The latent image cross-sectional area value can be calculated.
[0084]
Here, FIG. 4 shows the carrier generation amount when a two-dot line electrostatic latent image is formed in the main scanning direction by simultaneous light scanning, and the two-dot line electrostatic latent image in the main scanning direction by sequential scanning. 7 is a graph showing a difference from the amount of generated carriers in the case of forming. FIG. 5 is a graph showing an electric field intensity distribution when an electrostatic latent image is formed by simultaneous light scanning and an electric field intensity distribution when the same electrostatic latent image is formed by sequential scanning.
[0085]
As can be seen from FIG. 4, the amount of carriers generated during the optical scanning of the second line during sequential exposure is smaller than the amount of carriers generated during the optical scanning of the second line during simultaneous exposure. In the sequential exposure, since the first line is optically scanned and then the next second line is optically scanned, carriers have already been generated when the second line is optically scanned. Decrease. On the other hand, in the simultaneous exposure, the first line and the second line are simultaneously exposed and scanned, so that the carrier generation amount does not decrease, and the carrier generation amount increases in the simultaneous exposure than in the sequential exposure. In addition, the number of carriers in a region where the exposure overlaps (shaded portion) increases.
[0086]
As described above, the carrier recombination depends on the carrier generation amount (see the equations (2) and (3)). Therefore, the carrier recombination is larger in the simultaneous exposure than in the sequential exposure in which the carrier generation amount is larger than in the sequential exposure. Often occur.
[0087]
For this reason, although the amount of carriers generated during simultaneous exposure is larger than that during sequential exposure, as shown in FIG. 5, the field intensity distribution during simultaneous exposure is narrower and smaller than the field intensity distribution during sequential exposure. An irregular phenomenon occurs.
[0088]
By visualizing the electrostatic latent image in which the reciprocity irregular phenomenon has occurred as described above with toner, fluctuations in the line width periodically occur and are observed as banding.
[0089]
The higher the writing density, the higher the number of generated carriers and the greater the effect of carrier recombination with higher image quality. The higher the writing density, the more banding occurs. It is feared that.
[0090]
Here, FIG. 6 shows a multi-beam scanning method with a writing density of 1200 dpi, and a simultaneous exposure in the case where an electrostatic latent image is formed by exposing a 2-dot main scanning line by two methods of simultaneous exposure and sequential exposure. 9 is a graph showing a relationship between a difference between a line cross-sectional area and a line cross-sectional area by sequential exposure (line cross-sectional area difference) and perceived banding.
[0091]
The perceived banding is an evaluation scale for quantitatively evaluating the banding. The magnitude of the perceived banding is shown below as a function of the average brightness L of the image, the spatial frequency f, and the brightness amplitude ΔL at the spatial frequency (9) ) Is calculated by the equation.
[0092]
(Equation 6)

[0093]
Here, a, b, c, d, e, g: constant, L: average brightness, f: spatial frequency, ΔL: brightness amplitude at spatial frequency
As can be seen from FIG. 6, the effect of banding becomes smaller as the line cross-sectional area difference decreases.
[0094]
By the way, the perceived banding value that is not perceived as banding is set to 0.2 or less. According to FIG. 6, the line cross-sectional area difference at which the perceived banding value is 0.2 or less is 2.7 × 10 ^-6 It can be seen that:
[0095]
That is, according to FIG. 6, the line cross-sectional area difference is 2.7 × 10 ^-6 It can be seen that banding can be prevented by suppressing the following.
[0096]
Here, FIG. 7 is a graph showing the relationship between the beam overlap length in the sub-scanning direction and the line cross-sectional area difference. In FIG. 7, the beam superimposed length in the sub-scanning direction indicates the length of the portion where the beam spot diameters overlap each other in the sub-scanning direction. The beam spot diameter is 1 / e of the maximum beam intensity. ² Expressed by diameter.
[0097]
As the overlap length decreases, the overlap exposure portion decreases. As described above, the amount of carrier generation decreases as the overlapped exposure portion decreases. For this reason, it is considered that by reducing the beam superimposed length in the sub-scanning direction, carrier recombination during simultaneous exposure is unlikely to occur, and the line cross-sectional area difference is reduced.
[0098]
According to FIG. 6, the cross-sectional area difference where the occurrence of banding is suppressed is 2.7 × 10 ^-6 μm ² Therefore, from FIG. 7, it is possible to suppress the occurrence of banding by setting the overlap length in the sub-scanning direction to 21 μm or less.
[0099]
By the way, if the superimposed length is equal to or less than the minimum pixel, even if there is a superimposed portion, the superimposed portion will not be recognized as banding.
[0100]
Since the laser printer 1 according to the present embodiment has a target resolution of 1200 dpi, the minimum pixel size that can be resolved at this resolution can be obtained by the calculation of Expression (10).
[0101]
(Equation 7)

[0102]
According to equation (10), the minimum resolvable pixel size is 21.17 × 10 when the writing resolution is 1200 dpi. ^-6 μm. That is, the superimposed length in the sub-scanning direction is 21.17 × 10 ^-6 By setting the thickness to not more than μm, banding can be suppressed.
[0103]
Next, a second embodiment of the present invention will be described with reference to FIGS. Note that the same parts as those in the first embodiment are denoted by the same reference numerals, and description thereof will be omitted.
[0104]
The number of channels of the semiconductor laser array 19 in the laser printer 1 of the present embodiment is four, and the emission wavelength of light emitted from each of the light emitting units ch1, ch2, ch3, and ch4 is set to 780 nm. The multi-beam scanning device 18 including such a semiconductor laser array 19 irradiates a 35-70 μm beam spot on the surface of the photoconductor 13, and has a laser scanning speed of 1983.87 m / s (image frequency: 92.5 MHz) and a linear speed. Optical scanning (exposure) is performed at 0.362 m / s (scan frequency: 4275.6 Hz) and writing density: 1200 dpi. Exposure energy required for exposure is 7 erg / cm ² It is.
[0105]
The quantum efficiency η of the photoconductor 13 is 5.6 × 10 ^-6 ・ E ^0.7022 The absorption coefficient of the photoconductor 13 is set to 0.65, and the thickness of the photoconductor 13 is set to 30 μm.
[0106]
In addition, the initial charging potential of the surface of the photoconductor 13 by the charging roller 14 is set to -800V.
[0107]
Here, FIG. 8 is a graph showing the relationship between the diameter of the beam spot in the sub-scanning direction and the superimposed length of the electrostatic latent image line in the sub-scanning direction in the laser printer 1 of the present embodiment. According to FIG. 8, by making the diameter of the beam spot in the sub-scanning direction equal to or less than 42 μm, the superimposed length in the sub-scanning direction is equal to or less than the minimum pixel (one pixel) as in the first embodiment. Can be.
[0108]
Meanwhile, as can be seen from FIG. 8, the beam superimposed length in the sub-scanning direction decreases as the beam spot diameter decreases.
[0109]
Here, FIG. 9 is a graph showing the relationship between the diameter of the beam spot in the sub-scanning direction and the line width. As can be seen from FIG. 9, when the diameter of the beam spot in the sub-scanning direction becomes 42 μm or less, the line widths in the simultaneous exposure and the sequential exposure substantially match. Thus, in the laser printer 1 having a writing resolution of 1200 dpi, banding can be suppressed by setting the diameter of the beam spot in the sub-scanning direction to 42 μm or less.
[0110]
Next, a third embodiment of the present invention will be described with reference to FIG.
[0111]
The number of channels in the semiconductor laser array 19 of the multi-beam scanning device 18 provided in the laser printer 1 of the present embodiment is two, and the emission wavelength of light emitted from each of the light emitting units ch1 and ch2 is set to 655 nm.
[0112]
The multi-beam scanning device 18 including such a semiconductor laser array 19 irradiates a beam spot of 70 μm in the main scanning direction and 85 μm in the sub-scanning direction on the surface of the photoconductor 13, and has an image frequency of 64.3 MHz and a linear velocity of 79 mm / s. Optical scanning is performed at a writing density of 1200 dpi. Exposure energy required for exposure is 7 erg / cm ² It is.
[0113]
The superimposed length in the sub-scanning direction on the surface of the photoconductor 13 optically scanned by the multi-beam scanning device 18 is set to one pixel or less.
[0114]
In addition, the quantum efficiency η of the photoconductor 13 of the present embodiment is 2.83 × 10 ^-5 ・ E ^0.5933 The absorption coefficient of the photoconductor 13 is set to 0.7, and the thickness of the photoconductor 13 is set to 28 μm.
[0115]
By the way, if the electric field strength (charging potential / carrier moving layer thickness) obtained from the charging potential of the photoreceptor 13 and the thickness (film thickness) of the carrier moving layer is larger than 30 V / μm, background fouling is likely to occur. It is said. The photoconductor 13 has a quantum efficiency η of 0.4 <η <1 for preventing the background contamination and forming a good image. The quantum efficiency η of the photoconductor 13 of the present embodiment is 2.83 × 10 ^-5 ・ E ^0.5933 Therefore, it is set in the range of 0.4 <η <1 described above.
[0116]
Here, FIG. 10 is a graph showing light attenuation characteristics of the photoconductor 13. As can be seen from FIG. 10, the attenuation characteristic when writing is performed on the photosensitive member 13 by multi-beam scanning sequentially and the attenuation characteristic when writing is performed by simultaneous light scanning have different curves. The difference between the attenuation characteristic at the time of writing by sequential optical scanning and the attenuation characteristic at the time of writing by simultaneous optical scanning causes a difference in dot width and line width of the electrostatic latent image, which causes banding.
[0117]
Since the laser printer 1 of the present embodiment uses the multi-beam scanning device 18 in which the beam superposition length in the sub-scanning direction is set to one pixel or less, the quantum efficiency η is in the range of 0.4 <η <1. In the case of using the photoreceptor 13 set as described above, it is possible to prevent the electrostatic latent image charge from being reduced at the time of simultaneous light scanning, and to suppress the occurrence of banding.
[0118]
Next, a fourth embodiment of the present invention will be described with reference to FIG.
[0119]
The laser printer 1 of the present embodiment has a correlation coefficient R when the input / output characteristics are linearly regressed. ² It has input / output characteristics set to achieve a value of 0.97 or more.
[0120]
Further, the multi-beam scanning device 18 emits a beam whose superimposed length in the sub-scanning direction on the surface of the photoconductor 13 is equal to or less than the minimum pixel. The writing density of the multi-beam scanning device 18 of the present embodiment is 1200 dpi, so that the superimposed length in the sub-scanning direction on the surface of the photoconductor 13 is set to 21.17 μm (one pixel) or less. I have.
[0121]
Incidentally, higher resolution and improved gradation reproducibility in the laser printer 1 are determined by conditions such as solid density, smoothness of input / output characteristics, and skipping or crushing of gradation.
[0122]
That is, in the laser printer 1 which has a sufficient solid density, smooth input / output characteristics of the laser printer 1 and less gradation skipping and crushing, it is possible to realize image formation with high resolution and good tone reproduction. it can.
[0123]
FIG. 11 is a graph showing an example of the input / output characteristics of the laser printer 1 that forms such a high-resolution and high-quality image. As shown by the dotted line in FIG. 11, the input / output characteristics of the laser printer 1 are represented by a correlation coefficient R at the time of linear regression of the input / output characteristics. ² When the value is low, the influence of the difference in the line width of the electrostatic latent image formed by the multi-beam is not affected, but as the correlation coefficient increases, the influence of the difference in the line width increases. In the input / output characteristics shown in FIG. ² When the value is 0.97 or more, the influence of the difference in line width increases.
[0124]
In the present embodiment, in the multi-beam scanning device 18 that realizes the writing density of 1200 dpi, the superposition length in the sub-scanning direction on the surface of the photoconductor 13 is set to the minimum pixel or less, that is, 21.17 μm or less. Number R ² Even in the laser printer 1 having a value of 0.97 or more, banding can be suppressed.
[0125]
Next, a fifth embodiment of the present invention will be described with reference to FIGS.
[0126]
The number of channels in the semiconductor laser array 19 of the multi-beam scanning device 18 provided in the laser printer 1 of the present embodiment is two.
[0127]
In the case 15a of the developing device 15 of the present embodiment, a toner having a particle size of 6.5 μm is held. The content of the colorant (the degree of coloring of the toner) of the toner in the case 15a is set in the range of 5 to 15 wt%. In the present embodiment, the colorant content of the toner is set to be 6%.
[0128]
The developing device 15 has a toner adhesion amount M / A in a region (solid region) equal to or higher than the critical electric field intensity, where 0.2 ≦ M / A ≦ 0.8 mg / cm. ² Is developed so as to satisfy the range of By performing the development with the toner adhesion amount M / A in the solid region within the above-described range, one layer of toner is adhered to the photoconductor surface. Hereinafter, development under such conditions is referred to as low M / A development. In this embodiment, the toner adhesion amount M / A in the solid region is 0.47 mg / cm. ² The development is performed so as to satisfy the following.
[0129]
The amount of toner adhering to the photoconductor 13 by performing the low M / A development corresponds to the line width of the electrostatic latent image when cut at the critical development electric field intensity.
[0130]
Here, FIG. 12 is a graph showing the relationship between the electrostatic latent image line width and the perceived banding in the sub-scanning direction. In FIG. 12, the difference (line width difference) between the electrostatic line width at the time of simultaneous exposure and the line width at the time of sequential exposure is shown as Δ line width. According to FIG. 12, no banding occurs when the difference (line width difference) between the line width of the electrostatic latent image during simultaneous exposure and the line width of the electrostatic latent image during sequential exposure is 4.2 μm. I understand. Here, since the number of channels in the semiconductor laser array 19 is 2, the line width of the image is about 42 μm (21.17 μm × 2), and 4.2 μm corresponds to about 10% of the image line width. I understand.
[0131]
FIG. 13 is a graph showing a relationship between a difference (line width difference) between an electrostatic line width at the time of simultaneous exposure and a line width at the time of sequential exposure, and a superposition length in the sub-scanning direction. According to FIG. 13, in the laser printer 1 employing the low M / A development, it can be seen that the superimposed length in the sub-scanning direction where banding does not occur is 42 μm or less.
[0132]
As described above, in the laser printer 1 employing the low M / A development, banding can be suppressed if the overlapping length is 42 μm or less, and compared with a laser printer not employing the low M / A development. The allowable value of the beam superposition length for reducing banding is widened. This makes it easier to manage the beam superimposed length as compared with a laser printer that does not employ low M / A development.
[0133]
By the way, it is understood that the numerical value of the superimposed length of 42 μm is almost twice the minimum pixel that can be resolved at a resolution of 1200 dpi. That is, by adopting the low M / A development, the allowable value of the beam superposition length for reducing banding can be expanded to twice the minimum pixel.
[0134]
Next, a sixth embodiment of the present invention will be described with reference to FIG. The present embodiment is a laser printer 1 similar to the laser printer of the fifth embodiment.
[0135]
FIG. 14 is a graph showing the relationship between the beam spot diameter and the superimposed length in the sub-scanning direction in the laser printer 1 according to the present embodiment. According to FIG. 14, the beam spot diameter in the sub-scanning direction to achieve 21.17 × 2 = 42.33 μm or less, which is twice the minimum resolvable pixel, with the multi-beam scanning device 18 having a writing density of 1200 dpi. Is 63 μm or less.
[0136]
As described in the second embodiment, when performing optical scanning at a writing density of 1200 dpi with the laser printer 1 not employing the low M / A development, the beam spot diameter in the sub-scanning direction is set to 42 μm or less. Although it is necessary, by adopting the low M / A development, the allowable value of the beam spot diameter in the sub-scanning direction for reducing banding can be increased to 63 μm. This makes it easier to manage the beam superimposed length as compared with a laser printer that does not employ low M / A development.
[0137]
Next, a seventh embodiment of the present invention will be described with reference to FIG.
[0138]
In the present embodiment, the difference between the line width at the time of sequential exposure and the line width at the time of simultaneous exposure is reduced by weakening the exposure amount at the time of sequential exposure instead of adjusting the superposition length of the beam in the sub-scanning direction. Let it.
[0139]
FIG. 15 shows the exposure amount distribution when the exposure amount of the sequentially exposed portion is set to 100% and the exposure amount where the exposure amount of the sequentially exposed portion is set to 90% in the laser printer 1 of the present embodiment. 3 is a graph showing an exposure amount distribution. In the present embodiment, the light emission amounts of the light-emitting portions ch1 and ch4 at both ends in the sub-scanning direction of the semiconductor laser array 19 are reduced as compared with the light-emitting portions ch2 and ch3 at the center, thereby sequentially exposing the light exposure portions. Was adjusted. As can be seen from FIG. 15, the beam superimposed length in the sub-scanning direction when the light emission amount of the light emitting units ch1 and ch4 is set to 90% (exposure amount 90%) with respect to the central light emitting units ch2 and ch3 is as follows. Although the light emission amount of the light-emitting portions ch1 and ch4 is set to 100% (exposure amount 100%) with respect to the central light-emitting portions ch2 and ch3, the beam superimposed length in the sub-scanning direction is not changed. It can be seen that the exposure amount (superimposed exposure amount) in the superimposed portion when the exposure is performed at the amount of 90% is lower than the superimposed exposure amount when the exposure is sequentially performed at the exposure amount of 100%.
[0140]
Here, FIG. 16 is a graph showing the relationship between the amount of superimposed exposure and perceived banding. As can be seen from FIG. 16, the perceived banding is reduced by reducing the superimposed exposure amount during the sequential exposure.
[0141]
According to FIG. 16, suppression of perceptual banding is expected by reducing the exposure amount and reducing the superimposed exposure amount.
[0142]
However, if the exposure amount is too weak, the line width of the electrostatic latent image formed by the sequential exposure becomes smaller than the line width of the electrostatic latent image formed by the simultaneous exposure. There is a concern that the difference between the two will increase, and the perception will be perceived as banding.
[0143]
That is, it is necessary to grasp a change in perceived banding due to a change in the exposure amount during the sequential exposure, and control the exposure amount so that the banding phenomenon does not occur due to the superimposed exposure.
[0144]
Here, FIG. 17 is a graph showing the relationship between the change in the perceived banding due to the change in the exposure amount during the sequential exposure. As can be seen from FIG. 17, the light emission from the light emitting units ch1, ch2, ch3, and ch4 is such that the light emission amount of the light emitting units ch1 and ch4 is 90% (the exposure amount is 90%) with respect to the central light emitting units ch2 and ch3. It can be seen that when the light emission amount is controlled, perceptual banding is effectively suppressed.
[0145]
Thus, the light emission amounts of the light emitting units ch1, ch2, ch3, and ch4 are controlled such that the light emission amounts of the light emitting units ch1 and ch4 are 90% (the exposure amount is 90%) with respect to the central light emitting units ch2 and ch3. By doing so, it is possible to suppress the banding phenomenon that occurs due to the overlapping exposure in the multi-beam scanning device 18 that performs the adjacent scanning, and it is possible to form a high-quality image.
[0146]
Next, an eighth embodiment of the present invention will be described with reference to FIGS.
[0147]
This embodiment shows an example of application to a digital copying machine as a copying apparatus.
[0148]
FIG. 18 is a longitudinal sectional view showing the digital copying machine of the present embodiment. The digital copying machine 50 according to the present embodiment reads a document with a color scanner device 51 as an image reading device that reads an image of a document, a document feeding device 52 provided above the scanner device 51, and a color scanner device 51. A color laser printer 53 is provided as an image forming apparatus for forming an image based on image data of a document by an electrophotographic method.
[0149]
First, the color scanner device 51 will be described. The color scanner device 51 includes a scanner case 55 having a contact glass 54 on which an original is set.
[0150]
Inside the scanner case 55, a first traveling body 58 on which an exposure lamp 56 for emitting light and a mirror 57 are mounted, and a second traveling body 61 on which two mirrors 59 and 60 are mounted are provided. The exposure lamp 56 and the mirrors 57, 59, 60 are arranged with the longitudinal direction parallel to the main scanning direction. The first and second traveling bodies 58 and 61 are provided so as to be able to reciprocate in the sub-scanning direction (the horizontal direction in FIG. 1) along the contact glass 54, and have a 2: 1 speed ratio by a stepping motor (not shown). It is run by.
[0151]
The color scanner device 51 causes the exposure lamp 56 to emit light and reciprocates the first and second traveling bodies 58 and 61 along the contact glass 54, and the width of the exposure lamp 56 in the main scanning direction and the movement of the exposure lamp 56. The document is exposed and scanned in a range determined by the distance.
[0152]
The light obtained by exposing and scanning the original is guided by mirrors 57, 59, and 60, and forms an image on a color image sensor 64 mounted on an image reading substrate 63 via an imaging lens 62.
[0153]
The image reading substrate 63 photoelectrically converts the analog image data of the exposure light imaged on the color image sensor 64 via the original to generate digital image data.
[0154]
The digital image data generated by the image reading board 63 is subjected to various known image processing such as, for example, shading correction processing, MTF correction processing, and γ correction processing, and is provided to the subsequent processing.
[0155]
Next, the document feeder 52 will be described. The document feeder 52 is provided with a document table 65 on which sheet-shaped documents are set. When there are a plurality of documents, the documents are set in a state of being stacked on the document table 65. In FIG. 12, a document feed path 67 extending from the paper feed port 66 to a discharge port 73 via a reading position on a document table is communicated with a paper feed port 66 provided at a left end position of the document table 65. .
[0156]
Although not particularly shown, at the communication position between the document transport path 67 and the paper feed port 66, a call roller for feeding the document set on the document table 65 into the document transport path 67, and a call roller for feeding the document set into the document transport path 67. A separation mechanism or the like for separating the sent originals one by one is provided.
[0157]
In the document transport path 67, a transport belt 71 that is wound around a plurality of transport rollers 70 and transports the document by being rotated by the transport rollers 70 is provided. The transport belt 71 is provided so as to cover the entire surface of the contact glass 54. The transport belt 71 transports the original, presses the original on the contact glass 54 against the contact glass 54, and lifts the original from the contact glass 54. Plays the function of preventing. The call roller, the separation mechanism, and the plurality of transport rollers are rotationally driven by a motor (not shown).
[0158]
In addition, the document conveying path 67 is provided with a stop claw (not shown) for stopping the document conveyed on the contact glass 54 at the reading position. When the exposure scanning is completed, the stop claw is displaced so as to open the document conveying path 67, thereby allowing the document to be discharged. The document discharged from the discharge port 73 is stacked on a discharge table 74.
[0159]
The original feeding device 52 feeds the original into the original conveying path 67 by a call roller, and conveys only one original separated one by one by a separation mechanism onto the contact glass 54 by the conveying belt 71. When the document reaches the reading position, the rotation of the transport roller 70 is stopped. As a result, the document is positioned on the contact glass 54. The size of the document in the transport direction can be obtained, for example, by providing a sensor (not shown) in the document transport path and detecting the timing of document detection by this sensor, and by driving the motor based on this detection timing. The document can be accurately positioned on the contact glass 54.
[0160]
When the above-described exposure scanning of the original by the color scanner device 51 is completed in a state where the original is positioned on the contact glass 54, the original feeding device 52 resumes the driving of the transport roller 70, and The document is conveyed to a discharge table 74. Since the conveyance belt 71 is provided so as to cover the entirety of the contact glass 54, by adjusting the drive timing of the call roller and the separation mechanism, the document on the contact glass 54 is conveyed to the discharge table 74 at the same time. The next document can be fed.
[0161]
Next, the color laser printer 53 will be described. In the color laser printer 53, a recording medium guide path 80 is formed from a paper supply section 75 to a paper output tray 79 via a vertical transport section 76, a printer engine 77, a fixing section 78 and the like.
[0162]
The paper feed unit 75 separates the first and second paper feed trays 84 and 85 that hold the recording media 3 in a stacked state and the stacked recording media 3 provided in each of the paper feed trays 84 and 85 into one sheet. Paper feed mechanisms 87 and 88 for feeding the paper to the vertical transport section. The paper feed mechanisms 87 and 88 are each composed of a pickup roller 90, a reverse roller (not shown), and the like, and separate and record the recording media 3 stacked and held on each of the paper feed trays 84, 85, and 86 one by one. Send out to the media guideway. The pickup roller 90 and the reverse roller are driven to rotate by a motor (not shown).
[0163]
The vertical transport section 76 is provided with a plurality of transport rollers 92 for transporting the recording medium 3 fed from the paper feed trays 84 and 85 to the printer engine 77, respectively.
[0164]
The printer engine 77 is a tandem-type printer engine that forms a color image of each color of Y, M, C, and K on an intermediate transfer belt 94 as an object to be transferred wound around a plurality of rollers 93. Photoconductors 95 (95Y, 95M, 95C, 95K) provided for each of the colors Y, M, C, and K are provided. Around each photoconductor 95, a charging roller 96 as a charging device for charging the surface of each photoconductor 95, a multi-beam scanning device 97 for optically scanning the corresponding photoconductor 95 based on predetermined color image data, A developing device 98 for attaching a toner of a predetermined color to the electrostatic latent image written on the surface of the photoconductor 95 by the beam scanning device 97, and a toner image of each color formed on the surface of each photoconductor 95 is transferred to the intermediate transfer belt 94. An intermediate transfer device 99, a transfer unit 100 as a second transfer device for transferring a single color or a plurality of color toner images formed on the intermediate transfer belt 94 to the recording medium 3 conveyed in the recording medium guide path 80, A cleaner 101 for removing the toner remaining on each photoconductor 95 and a cleaner (not shown) for removing the toner remaining on the intermediate transfer belt 94. And it has a toner or the like.
[0165]
The multi-beam scanning device 97 of this embodiment writes an electrostatic latent image corresponding to each color by optically scanning four lines on each photoconductor 95. The multi-beam scanning device 97 has a voltage value V _L Is written on the surface of the photoconductor 95.
[0166]
Since the multi-beam scanning device 97 has the same configuration as the multi-beam scanning device 18 of the first embodiment, it is not shown and described, but includes a 4-ch semiconductor laser array corresponding to each photoconductor 95. ing. The optical components for guiding the beam emitted from each semiconductor laser array to the corresponding photoconductor 95 may be provided individually for each semiconductor laser array, may be shared, or may be partly provided. May be shared.
[0167]
As shown in FIG. 19, the intermediate transfer device 99 according to the present embodiment is a first transfer device that is opposed to each photoconductor 95 via an intermediate transfer belt 94 and applies a transfer bias to the photoconductor 95. , And an applied voltage member 99b disposed upstream of each intermediate transfer bias roller 99a in the rotation direction of the intermediate transfer belt 94. The applied voltage member 99b has the same polarity as the charging polarity of the photoconductor 95, and has a voltage value V of an electrostatic latent image written on the surface of the photoconductor 95. _L A voltage having an absolute value larger than that of the intermediate transfer belt 94 is applied.
[0168]
In transferring the toner image formed on the surface of the photoconductor 95 to the intermediate transfer belt 94, the above-described voltage is applied to the intermediate transfer belt 94 by the applied voltage member 99b, and then the transfer bias is applied by the intermediate transfer bias roller 99a. Apply. This prevents the toner image from being transferred to the intermediate transfer belt 94 (pre-transfer) before entering the transfer area by the intermediate transfer bias roller 99a, and prevents the toner image from being transferred from each photoconductor 95 to the intermediate transfer belt 94. In addition, it is possible to prevent the toner image from being disturbed by dust and the like, and to form a stable image irrespective of variation over time and variations in the developer.
[0169]
A full-color image is formed on the intermediate transfer belt 94 by transferring the toner image formed by each photoconductor 95. The full-color image formed on the intermediate transfer belt 94 is transferred to a recording medium by the transfer unit 100.
[0170]
According to the digital copying machine 50 of the present embodiment, it is possible to prevent banding from occurring in the electrostatic latent image formed on the surface of the photoconductor 95 by setting the superimposed length in the sub-scanning direction to the minimum pixel or less. Therefore, it is possible to prevent the quality of the toner image formed on the surface of the photoconductor 95 from deteriorating due to banding, and to transfer the toner image formed on the surface of each photoconductor 95 onto the intermediate transfer belt 94 well. Thus, an image faithful to the original image read by the color scanner device 51 can be copied and formed.
[0171]
【The invention's effect】
According to the image forming apparatus of the first aspect of the present invention, when the superimposed length of the scanning line formed on the surface to be scanned in the sub-scanning direction is set to be equal to or less than the minimum pixel, writing in high resolution is performed. It is also possible to prevent the occurrence of banding due to superimposed exposure without being affected by resolution, and to prevent the occurrence of banding due to superimposed exposure without being affected by resolution even when writing at high resolution. Thus, a high-quality image faithful to the original image can be formed.
[0172]
According to the second aspect of the present invention, in the image forming apparatus according to the first aspect, when the resolution at which 21.17 μm or less is the minimum pixel or less is 1200 dpi, the superimposed length in the sub-scanning direction is 21 μm or less. By doing so, it is possible to prevent banding from occurring due to the superimposed exposure, and to form a high-quality image faithful to the original image.
[0173]
According to the third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the diameter of the beam spot in the sub-scanning direction is set to 42 μm or less, so that the beam spot size is reduced as the beam spot diameter is reduced. The superimposed length of the spot in the sub-scanning direction can be made the minimum pixel or less, and banding due to superimposed exposure can be prevented, and a high-quality image faithful to the original image can be formed.
[0174]
According to a fourth aspect of the present invention, in the image forming apparatus of the first, second or third aspect, the quantum efficiency η of the photoconductor is set to 0.4 < By setting the value in the range of η <1, it is possible to prevent occurrence of banding and background contamination, and to form a high-quality and good image faithful to the original image.
[0175]
According to the fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, the correlation coefficient R at the time of linear regression of the input / output characteristics. ² By setting the value to 0.97 or more, the influence of banding caused by the difference between the line width when optical scanning is simultaneously performed on the surface to be scanned once and the line width when optical scanning is sequentially performed twice is performed. Banding can be prevented from occurring even under conditions that are easily affected.
[0176]
According to a sixth aspect of the present invention, in the image forming apparatus according to any one of the first to fifth aspects, the toner adhesion amount M / A in a region having a critical electric field strength or more is 0.2 ≦ M / A ≦ 0. .8mg / cm ² By performing low M / A development in which one layer of toner adheres to the surface of the photoreceptor, the superposed length of the scanning line formed on the surface to be scanned in the sub-scanning direction is twice the minimum pixel. It is possible to prevent the occurrence of banding due to superimposed exposure, regardless of the resolution, as compared with an image forming apparatus that does not perform low M / A development Can be.
[0177]
According to the seventh aspect of the present invention, in the image forming apparatus according to the sixth aspect, by setting the diameter of the beam spot in the sub-scanning direction to 63 μm or less, the size of the beam spot that decreases with the reduction of the beam spot diameter is reduced. The superimposition length in the sub-scanning direction can be made the minimum pixel or less, and the occurrence of banding due to superimposition exposure can be prevented without being affected by the resolution as compared with an image forming apparatus that does not perform low M / A development. The range of the diameter of the beam spot in the sub-scanning direction can be widened.
[0178]
According to the image forming apparatus of the present invention, the light amount for optically scanning both ends in the sub-scanning direction on the surface to be scanned is made smaller than the light amount for optically scanning other than both ends in the sub-scanning direction. It is possible to reduce the difference between the line width at the time of sequential exposure and the line width at the time of simultaneous exposure, prevent the occurrence of banding due to superimposed exposure, and form a high-quality image faithful to the original image. .
[0179]
According to the ninth aspect of the present invention, in the image forming apparatus of the eighth aspect, the light amount for optically scanning both ends in the sub-scanning direction on the surface to be scanned and the light amount for optically scanning other than the both ends in the sub-scanning direction. , The difference between the line width at the time of sequential exposure and the line width at the time of simultaneous exposure can be reliably reduced, banding due to superimposed exposure can be prevented, and the original image can be prevented. And a high-quality image can be formed.
[0180]
According to a tenth aspect of the present invention, in the image forming apparatus according to any one of the first to ninth aspects, prior to the transfer of the toner to the transfer target by the first transfer device, the charging polarity of the photoconductor is set. And the potential V of the electrostatic latent image formed on the surface of the photoreceptor. _L By applying a voltage having an absolute value larger than that of the first transfer device, the toner on the photosensitive member is pre-transferred to the transferred member or discharged to the toner on the photosensitive member before the toner is transferred by the first transfer device. Therefore, it is possible to prevent the occurrence of banding due to the superimposed exposure, to prevent the image dust, and to form a stable image irrespective of variation over time and variations in the properties of the developer.
[0181]
According to the copying apparatus of the eleventh aspect, the image reading apparatus has the function of the invention of any one of the first to tenth aspects and can form a high-quality image. An image faithful to the original image can be formed.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view showing a laser printer according to a first embodiment of the present invention.
FIG. 2 is a perspective view showing a multi-beam scanning device.
FIG. 3 is a graph showing a correlation between a line diameter and a field intensity of an electrostatic latent image in a sub-scanning direction.
FIG. 4 shows the amount of generated carriers when a two-dot line electrostatic latent image is formed in the main scanning direction by simultaneous light scanning, and forms a two-dot line electrostatic latent image in the main scanning direction by sequential light scanning. 6 is a graph showing a difference from a carrier generation amount in the case of performing the operation.
FIG. 5 is a graph showing an electric field intensity distribution when an electrostatic latent image is formed by simultaneous optical scanning and an electric field intensity distribution when the same electrostatic latent image is sequentially formed by optical scanning.
FIG. 6 is a line cross-sectional area by simultaneous exposure when an electrostatic latent image is formed by exposing a 2-dot main scan line by two methods of simultaneous exposure and sequential exposure in a multi-beam scanning method with a writing density of 1200 dpi 6 is a graph showing a relationship between a difference between the line cross-sectional area and a line cross-sectional area due to sequential exposure (line cross-sectional area difference) and perceived banding.
FIG. 7 is a graph showing a relationship between a beam superimposed length in a sub-scanning direction and a line cross-sectional area difference.
FIG. 8 is a graph showing a relationship between a diameter of a beam spot in a sub-scanning direction and a superimposed length of an electrostatic latent image line in a sub-scanning direction in the laser printer according to the second embodiment of the present invention.
FIG. 9 is a graph showing a relationship between a diameter of a beam spot in a sub-scanning direction and a line width.
FIG. 10 is a graph showing light attenuation characteristics of a photoconductor of a laser printer according to a third embodiment of the present invention.
FIG. 11 is a graph showing an example of input / output characteristics in a laser printer according to a fourth embodiment of the present invention.
FIG. 12 is a graph showing a relationship between an electrostatic latent image line width and a perceived banding in a sub-scanning direction in the laser printer according to the fifth embodiment of the present invention.
FIG. 13 is a graph showing a relationship between a difference (line width difference) between an electrostatic line width at the time of simultaneous exposure and a line width at the time of sequential exposure, and a superposition length in the sub-scanning direction.
FIG. 14 is a graph showing a relationship between a beam spot diameter and a superimposed length in a sub-scanning direction in a laser printer according to a sixth embodiment of the present invention.
FIG. 15 shows an exposure amount distribution when the exposure amount of the sequentially exposed portion is set to 100% and the exposure amount of the sequentially exposed portion in the laser printer according to the seventh embodiment of the present invention. 9 is a graph showing an exposure amount distribution when exposure is performed by setting 90% to 90%.
FIG. 16 is a graph showing the relationship between the amount of superimposed exposure and perceived banding.
FIG. 17 is a graph showing a relationship between a change in perceived banding due to a change in an exposure amount during sequential exposure.
FIG. 18 is a longitudinal sectional view showing a digital copying machine according to an eighth embodiment of the present invention.
FIG. 19 is an enlarged view showing the intermediate transfer device.
[Explanation of symbols]
1 Image forming apparatus
3 Transferee
13 Photoconductor
14 Charging device
15 Developing device
16 Transfer device
18 Multi-beam scanning device
50 Copier
51 Image reading device
53 Image forming apparatus
94 Transfer object
95 Photoconductor
96 Charging device
97 Multi-beam scanner
98 Developing device
99a First transfer device
99b Voltage application member
100 second transfer device
ch1, ch2, ch3, ch4 Light source

Claims (11)

A photoreceptor,
A charging device for charging the photoconductor surface,
Multi-beam scanning for simultaneously performing optical scanning on the surface to be scanned by a plurality of light beams by deflecting and scanning light emitted from a plurality of light sources, respectively, with the surface of the photoconductor charged by the charging device as a surface to be scanned. Equipment and
A developing device that supplies toner to the surface of the photoconductor that has been optically scanned by the multi-beam scanning device;
A transfer device that transfers the toner supplied to the photoreceptor surface by the developing device to a transfer target body,
With
An image forming apparatus in which a superimposed length in the sub-scanning direction of a scanning line formed on a surface to be scanned by simultaneous light scanning by the multi-beam scanning device is equal to or less than a minimum pixel. 2. The image forming apparatus according to claim 1, wherein the superimposed length of the scanning lines in the sub-scanning direction is 21 [mu] m or less when the resolution is 1200 dpi. When the resolution is 1200 dpi and the diameter of the beam spot formed on the surface to be scanned by a light beam that simultaneously optically scans the surface to be scanned is 1 / e ² of the maximum beam intensity, 3. The image forming apparatus according to claim 1, wherein the diameter in the scanning direction is 42 μm or less. 4. The image forming apparatus according to claim 1, wherein the photoconductor has a quantum efficiency η set to 0.4 <η <1. The correlation coefficient R ² value during the linear regression of the input and output characteristics, the image forming apparatus according to any one of claims 1 is set to 0.97 or more 4. The developing device supplies a toner having a colorant content of 5 to 15 w% to the photoconductor, and an electric field intensity distribution formed on the photoconductor surface by optical scanning of the multi-beam scanning device. The toner adhesion amount M / A in a region equal to or higher than the critical electric field intensity which is an electric field intensity to which toner adheres according to the following condition: 0.2 ≦ M / A ≦ 0.8 mg / cm ^2. The image forming apparatus as described in the above. 7. The image forming apparatus according to claim 6, wherein, when the resolution is 1200 dpi, the diameter of a beam spot formed on the surface to be scanned by the light beam that simultaneously scans the surface to be scanned is 63 μm or less in a sub-scanning direction. A photoreceptor,
A charging device for charging the photoconductor surface,
Multi-beam scanning for simultaneously performing optical scanning on the surface to be scanned by a plurality of light beams by deflecting and scanning light emitted from a plurality of light sources, respectively, with the surface of the photoconductor charged by the charging device as a surface to be scanned. Equipment and
A developing device that supplies toner to the surface of the photoconductor that has been optically scanned by the multi-beam scanning device;
A transfer device that transfers the toner supplied to the photoreceptor surface by the developing device to a transfer target body,
With
An image forming apparatus, wherein at the time of simultaneous light scanning by the multi-beam scanning device, the light amount for optically scanning both ends in the sub-scanning direction on the surface to be scanned is lower than the light amount for optically scanning other than both ends in the sub-scanning direction. In the simultaneous light scanning by the multi-beam scanning device, the light amount for optically scanning both ends in the sub-scanning direction on the surface to be scanned is 90% of the light amount for optically scanning other than both ends in the sub-scanning direction. Item 10. The image forming apparatus according to Item 8. The multi-beam scanning device forms an electrostatic latent image with a potential _{VL on} the surface of the photoconductor,
The transfer device is a first transfer device that is arranged in contact with the photoconductor and transfers the toner supplied to the surface of the photoconductor by the developing device to a transfer target, and a transfer device that transfers the toner transferred onto the transfer target. A second transfer device for transferring the image onto a recording medium; and a second transfer device provided upstream of the transfer position of the first transfer device in the rotation direction of the photoconductor and having the same polarity as the charge polarity of the photoconductor and absolutely higher than the potential _VL. The image forming apparatus according to claim 1, further comprising: a voltage applying member that applies a voltage having a large value. An image reading device that reads an original image,
The image forming apparatus according to claim 1, wherein an image based on a document image read by the image reading device is formed on a transfer target body,
A copying apparatus comprising:

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