JP3624926B2 - Image forming apparatus - Google Patents

Description

【００３０】
表１の評価で、◎は非常に良好の判定で、ジッタは全く目立たずジッタによる画質低下がないサンプル、○は良好の判定で、ジッタは注意して観察すると目立つが画質低下には至らないサンプル、×は不良の判定で、ジッタがはっきりと目立ってしまい画質を低下させているサンプルである。
【００３１】
表１より、サンプルＤ、サンプルＣ、サンプルＢ、サンプルＡの順に画像チリ量が大きなると、これに対してジッタも目だたず良好な画像となることがわかる。これは、ジッタによる濃度変動が画像チリの存在により適度にぼかされているために、人の目が感じにくいためである。
【００３２】
このように、画像チリ量が所定量即ち１５μｍ以上であることにより、ジッタを目立たせることがなく良好な画像を得ることができる。
【００３３】
〔実施例２〕
本発明の画像形成装置は、スクリーン構造を伴った画像にも用いることができる。フルカラー画像では写真やイラストなどのピクトリアル画像を得ることが多く、この時、多彩な階調表現を要求されるため何らかのスクリーン構造を伴って画像形成を行う場合が多い。ここで、本発明の画像形成装置において、スクリーン構造によって画像形成を行った場合について説明する。
【００３４】
スクリーン構造には、網点（ドット）スクリーン・万線スクリーン・クロスラインスクリーン・同心円スクリーン・砂目スクリーン・これらを併せ持つハイブリッドスクリーンなどがあり、電子写真一般に用いられるのはドットスクリーン・万線スクリーン、或いは両者のハイブリッドスクリーンである。何れのスクリーン構造も、写真・イラスト等、多階調のピクトリアル画像を再現する場合には不可欠な構成であり、特に、中間調画像部では、スクリーン構造によって、階調性・色再現性が大きく変わってくるため、最終画像の画質を決める重要な要素である。
【００３５】
本発明の第２の実施例ではドットスクリーン構造を伴った画像を用いるが、ここで図３を用いてドットスクリーンについて説明する。図３は本発明の画像形成装置におけるドットスクリーンによる最終画像サンプルの拡大図である。画像を構成するドット２０６は周囲に画像チリ２０３伴って規則的に整列し、ドット２０６が整列する方向に対するドット中心間の距離がドットスクリーンのスクリーンピッチ２０７である。本発明の画像形成装置では、通常、解像度が２００ｄｐｉ相当のスクリーンピッチ１２７μｍであるドットスクリーンが用いられるが、画像濃度や印字サンプルの種類に応じてスクリーンピッチを変化させてもよい。画像濃度はドットの大小によって決まり、これにより階調表現がなされる。また、フルカラー印字で多色のドットが重なる場合には、ドットスクリーンにスクリーン角を設け、各色ドットの重なり合いによって生じるモアレを防ぐ場合もある。
【００３６】
本実施例において、スクリーンピッチは、解像度が２００ｄｐｉ相当の１２７μｍ、１５０ｄｐｉ相当の１７０μｍ、１００ｄｐｉ相当の２５４μｍとした。また、第１の実施例同様、転写電界及びトナーの流動性を変化させることにより、画像チリ量の異なるサンプルを作成し、画像評価した。
【００３７】
第１の実施例同様に、ジッタに対する画像チリの影響を評価するため故意に既知の周期的なジッタ発生させるために、感光体１０１に駆動を伝達する輪列系のギヤの一つには歯の一部に欠けがあるものを用い、定期的に振動を与えるようにし、この時の印字パターンは、ドットスクリーンで作成した５０％濃度の均一パターンとした。
【００３８】
また幅広い階調を持つ写真画像では、中間調画像部、特にハイライト部においてスクリーンの目立ちやすさであるドット感に対して画像チリがどう影響するか、また、画像チリ量を大きくすることで画像の滲み・写真画像細部の表現にどう影響するか評価した。
【００３９】
評価は第１の実施例に基づき、複数の被験者の判定の最多数判定による。表２は、本発明の画像形成装置でドットスクリーンによる画像判定結果の結果で、それぞれの判定がスクリーンピッチ、画像チリ量によってどう判定されるかを示した。
【００４０】
【表２】

【００４１】
表２中、ジッタについての評価は第１の実施例同様、◎は非常に良好の判定で、ジッタは全く目立たずジッタによる画質低下がないサンプル、○は良好の判定で、ジッタは注意して観察すると目立つが画質低下には至らないサンプル、×は不良の判定で、ジッタがはっきりと目立ってしまい画質を低下させているサンプルである。画像チリ量は、スクリーンピッチによらず、より大きい方がジッタが目立たず良好になるという判定である。第１の実施例同様に、画像チリが１３μｍと小さい時にはジッタがはっきりと目立ってしまい画質が低下している不良の判定であり、１５μｍ以上の時にはジッタは注意して観察すると目立つが画質低下には至らない良好の判定となる。よって、ドットスクリーン構造の最終画像に１５μｍ以上の画像チリがあれば、ジッタが存在しても目立たない良好な画像を得ることができる。これは、画像チリのないスクリーンでは、信号として認識される周期的なスクリーン即ちドットの中でジッタが現れると、ノイズとして認識し、人の目の感度は高くなり不快感を伴い、画質低下となるが、ドット周囲の濃度変動が画像チリの存在によって適度にぼかされていると、信号としての認識も曖昧となり、その中でノイズが発生しても人の目は感じにくくなるためである。
【００４２】
次に、写真画像において評価したスクリーンを構成するドット感についての判定であるが、表２中のドット感の判定は、◎はドット感が全くない非常に良好なサンプル、○は注意して観察するとドット感があるが良好なサンプル、△はドット感はあるが実用上問題はない許容サンプルである。
【００４３】
ドットスクリーンの目立ちやすさであるドット感はスクリーンピッチにも依存し、スクリーンピッチが小さくなる程ドット感がなくなり良好になるが、いずれのスクリーンピッチにおいても画像チリが大きいほど、ドット感がなく良好なサンプルとなる。本発明の画像形成装置で標準で用いられるスクリーンピッチ１２７μｍ（２００ｄｐｉ相当）では、画像チリ量が１３μｍと小さい場合でも良好なサンプルであり、画像チリ量が大きいほどドット感はなくなり、ドット感だけではなく、スクリーン角を設けるにも関わらず生じてしまうモアレや、ドットスクリーンに特有のロゼッタパターンが目立たず、良好な画像が得られる。
【００４４】
一方、画像チリ量が大きくなりすぎると画像全体が滲んで不明瞭な画像となってくる場合がある。この現象は全体が暗いシャドウ部よりも、画像の色や形を認識しやすい中間調画像部において顕著である。表２の滲みの判定は、◎は非常に明瞭の判定で、滲みが全くなくエッジが非常にはっきり表現されているサンプル、○は明瞭の判定で、やや滲んでいるがエッジは十分表現されているサンプル、×は不明瞭の判定で、滲みが大きくエッジがボケて表現されているサンプルである。画像チリ量の大きさと画像の明瞭さの関係はスクリーンピッチに依存し、スクリーンピッチが大きい場合には、画像チリ量が大きくても滲みという点では気にならず良好な画像となる。画像チリは、スクリーンピッチの１／２を超えない範囲では、中間調画像部においても隣接するドットに過剰に干渉することなく、明瞭で良好な画像を得ることができる。
【００４５】
このように、ドットスクリーン構造の画像においては、画像チリ量が所定量即ち１５μｍ以上であるとジッタを目立たせることがなく、良好な画像を得ることができる。また、スクリーンを構成するドットのドット感のない良好な画像を得ることができる。また、画像チリ量がスクリーンピッチの１／２を超えない範囲では、画像の滲みがなく明瞭で良好な画像を得ることができる。
【００４６】
〔実施例３〕
第３の実施例において、本発明の画像形成装置で万線スクリーン構造を伴った最終画像について説明する。まず図４を用いて万線スクリーンについて説明する。図４は本発明の画像形成装置における万線スクリーンによる最終画像サンプルの拡大図である。画像を構成するライン２０８は周囲に画像チリ２０３を伴って、露光主走査方向２１０に垂直な一本のラインが順次形成されるよう規則的に整列し、ラインの中心間の距離が万線スクリーンのスクリーンピッチ２０９である。本発明の画像形成装置では、通常、解像度が２００ｌｐｉ相当の１２７μｍのスクリーンピッチである万線スクリーンが用いられるが、画像濃度や印字サンプルの種類に応じてスクリーンピッチを変化させてもよい。万線スクリーンではラインの太さによって濃度が決まり、これにより階調表現がなされる。
【００４７】
また、フルカラー印字で多色の万線スクリーンによる色重ねでは、同じ位置にライン２０８が重なり、定着部１２０において熱溶融され、減法混色に基づき発色する。ところが万線スクリーンの形成が狙った場所から僅かでもずれると色の見え方が違ってくる、即ち色再現安定性が悪くなる場合がある。このようなレジストレーションのズレは、図１の電子写真プロセス部中、露光工程での位置精度や感光体１０１や中間転写体１０６の駆動系による振動や、精度誤差による速度変動などで生じることがある。
【００４８】
本実施例において、スクリーンピッチは、解像度が２００ｌｐｉ相当の１２７μｍ、１５０ｌｐｉ相当の１７０μｍ、１００ｌｐｉ相当の２５４μｍとした。また、第１の実施例同様、転写電界及びトナーの流動性を変化させることにより、画像チリ量の異なるサンプルを作成し、画像評価した。
【００４９】
万線スクリーンでのレジストレーションのずれに対する画像チリ２０３の影響を評価するため、通常の色重ねに対し故意にレジストレーションをずらして作成した複色の印字パターンも用意し、両者で色再現安定性にどう影響するか評価した。この時の印字パターンは、万線スクリーンで作成した５０％濃度の均一パターンであり、通常の各色のライン２０８が同じ位置で層になって重なるパターンと、これに対してレジストレーションをずらした画像で、色重ねの中の１色をスクリーンピッチの半分だけ露光主走査方向２１０にずれる（スクリーンの谷間にずれる）よう、設定した印字パターンにて評価した。また幅広い階調を持つ写真画像では、特にハイライト部での万線スクリーン構造のライン感が気にならないか、また、画像チリ量を大きくすることで画像の滲みにどう影響するのか評価した。
【００５０】
画像評価は第１の実施例に基づき、複数の被験者の判定の最多数判定による。表３は、本発明の画像形成装置で万線スクリーンによる画像判定結果で、それぞれの判定がスクリーンピッチ、画像チリ量によってどう判定されるかを示した。
【００５１】
【表３】

【００５２】
表３中、色再現安定性の判定は、◎は色再現安定性が完全に確保された非常に良好なサンプル、○は色再現安定性が確保された良好なサンプル、△は色再現安定性がやや悪いが確保され実用上問題はない許容サンプル、×は色再現安定性が悪い不良サンプルである。画像チリ量が小さくなるに従い、レジストレーションをずらした画像サンプルと通常のものとでは色の見え方が変わる、即ち色再現安定性が悪くなる。
【００５３】
これは、両者の混色の方法が異なるためである。通常の同じ位置で色重ねしたスクリーンでは、定着部１２０により複数の色の層が同位置で混色し、トナー層それぞれの分光反射率を波長毎にほぼ積算した曲線で表すことができる、減法混色に基づいて色再現される。これに対してレジストレーションがずれたスクリーンでは、各スクリーンが重ならずに並置し、各スクリーンからの反射光の混合即ち加法混色の一種である並置混色によって、色再現される。減法混色ではもとの色より暗い色となり、並置混色で得られる色はもとの色のほぼ中間の明度となるため、同量のトナーが被記録紙上に存在しても見え方が異なる、つまり色再現安定性が悪くなるのである。
【００５４】
ところが画像チリ量が大きい画像においては、同位置での色重ねでもレジストレーションがずれた色重ねでも、減法混色と並置混色とが混在し、定着部１２０でトナーが潰されつつ拡大されると殆どの領域で減法混色となるので、色重ね位置の変動に対しても常に色再現安定性が確保される。
【００５５】
本発明の画像形成装置で標準に用いられる２００ｌｐｉ相当の１２７μｍのスクリーンピッチでは、画像チリ量が１５μｍ以上あれば色再現安定性が実用上問題なく、画像チリ量が大きいほど色再現性が確保され、良好になる。スクリーンピッチが大きい場合にも同様に画像チリ量が１５μｍ以上では色再現安定性が実用上問題なく許容できるが、スクリーンピッチは小さい程色再現安定性は良好となる。
【００５６】
色重ねに関する以上の現象はドットスクリーンにおいても見られることであるが、ドットスクリーンのうちでスクリーン角を設けた場合には、本来並置混色と減法混色による色が混在した状態であり、画像チリ量の小さいサンプルでレジストレーションがずれても、万線スクリーン程は重大な色再現性の差となって現れてはこない。
【００５７】
また、トナーを複数層重ねた時の転写性の問題で、画像チリが小さい場合には各色トナーが層になり転写媒体からの距離によって転写性能が異なり、これにより画像チリの状態が大きく異なり、画像チリがない層と画像チリが広範囲に及んで転写される層とで面積率が違ってくるため、色再現安定性を確保することができない。例えば中間転写体１０６から被記録材１１３へ転写される２次転写時には、最も被記録材１１３に近いトナー層では画像チリ量が小さく、被記録材から遠くなるに従い広範囲に及んで転写され、被記録材から最も遠いトナー層の色相寄りの色みとなってしまう。これに対し、本来画像チリ量が大きい場合は、各色のトナー層で画像チリの状態があまり変わらず、常に色再現安定性が確保される。
【００５８】
次に、写真画像において評価したスクリーンを構成するライン感についての判定であるが、表３中のライン感の判定は、◎はライン感が全くない非常に良好なサンプル、○は注意して観察するとライン感があるが良好なサンプル、△はライン感はあるが実用上問題はない許容サンプルである。スクリーンの目立ちやすさであるライン感はドットスクリーン同様、スクリーンピッチに依存する部分が大きいが、いずれのスクリーンピッチにおいても画像チリが大きいほど、ライン感がなく良好なサンプルとなる。本発明の画像形成装置で標準で用いられるスクリーンピッチ１２７μｍ（２００ｄｐｉ相当）では、画像チリ量が１３μｍと小さい場合でも良好なサンプルであり、画像チリ量が大きいほどライン感はなくなり、ライン感だけではなく、万線スクリーン画像に固有の問題で、スクリーンを構成するラインに対して斜め方向に伸びる細線や、斜めにコントラスト画像のエッジが存在する部分で、スクリーンピッチに起因する階段状のぎざぎざが目立たず、良好な画像が得られる。
【００５９】
一方、第２の実施例であるドットスクリーン同様に、画像チリ量が大きくなりすぎると画像全体が滲んで不明瞭な画像となってくる場合がある。表３の滲みの判定は、◎は非常に明瞭の判定で、滲みが全くなくエッジが非常にはっきり表現されているサンプル、○は明瞭の判定で、やや滲んでいるがエッジは十分表現されているサンプル、×は不明瞭の判定で、滲みが大きくエッジがボケて表現されているサンプルである。画像チリ量の大きさと画像の明瞭さの関係はスクリーンピッチに依存し、スクリーンピッチが大きい場合には、画像チリ量が大きくても滲みという点では気にならず良好な画像となる。画像チリは、スクリーンピッチの１／２を超えない範囲では、中間調画像部においても隣接するラインに過剰に干渉することなく、明瞭で良好な画像を得ることができる。
【００６０】
また、不図示であるが、第２の実施例同様に、ジッタに対する画像チリの影響を評価したが、その結果は第２の実施例同様、画像チリ量が１５μｍ以上あればジッタを目立たせることがなく、良好な画像を得ることができ、その判定はスクリーンピッチを変化させても変わらない。
【００６１】
このように、万線スクリーン構造の画像においては、画像チリ量が所定量即ち１５μｍ以上であると常に色再現安定性を確保することができる。また、ハイライト部でもスクリーンを構成するラインのライン感がない良好な画像を得ることができる。また、画像チリ量がスクリーンピッチの１／２を超えない範囲では、画像の滲みがなく明瞭で良好な画像を得ることができる。
【００６２】
〔実施例４〕
画像形成装置において生じる画像チリは、例えば図３のドットスクリーンで示したような等方的に広がる画像チリばかりでなく、方向性を有して広がる場合がある。特に、線順次で画像形成を行うプロセスでは感光体１０１や中間転写体１０６などの駆動の影響を受けたり、現像・転写・定着の各プロセスで速度変動が生じたり振動を受けるなど何らかの原因で、画像チリは各画像形成装置固有の方向性を有する場合が多い。本発明の画像形成装置は、このように画像チリが方向性を有した場合についても用いることができる。
【００６３】
ここで、画像チリの方向性について、ドットスクリーン構造の場合を例にとり、図５を用いて説明する。図５は、本発明の画像形成装置における方向性を有する画像チリの一例の拡大図である。ドット２０６は周囲に画像チリ２０３を伴ってスクリーンピッチ２０７の間隔で被記録材の送り方向２１１に対して角度を有して整列するが、画像チリ２０３は画像形成装置の被記録材の送り方向２１１の方向に平行になるように広がる。一般に画像形成装置は、被記録材の送り方向２１１に散りが発生しやすい傾向にある。画像チリが方向性を有して広がると、例えば図５に示すようなドットスクリーンの場合、ドットの整列する方向（ドットが近接する方向）とドットの広がる方向が異なり角度を持っているので、画像チリ２０３が未印字部分へより広がることになる。
【００６４】
そこで本発明の第４の実施例では、図６に示すように前述の万線スクリーンを被記録材の送り方向２１１に対して垂直な一本のライン２０８となるような、横万線スクリーンとして画像形成させた。図６は、本発明の画像形成装置における横万線スクリーンによる最終画像の拡大図である。スクリーンピッチ２０９である横万線スクリーンにおいて、被記録材の送り方向２１１に画像チリ２０３が広がった時、画像を形成するライン２０８は未印字部分であるスクリーンの間へ万遍なく広がるような画像チリ２０３を有する。
【００６５】
本実施例において用いた画像形成装置は、固有の画像チリの方向性を有する。画像チリの方向は被記録材の送り方向に長く広がり、真円の微小ドットを印字すると被記録材の送り方向が長軸となる楕円形になり、その長軸と短軸との比率がほぼ１：０．７となる画像チリである。
【００６６】
本実施例においてスクリーンピッチは、解像度が２００ｌｐｉ相当の１２７μｍ、１５０ｄｐｉ相当の１７０μｍ、１００ｄｐｉ相当の２５４μｍとした。また、第１の実施例同様、転写電界及びトナーの流動性を変化させることにより、画像チリ量の異なるサンプルを作成し、画像評価した。
【００６７】
また、印字パターンはドットスクリーンで作成した５０％濃度の均一パターンで、第１の実施例同様、転写電界及びトナーの流動性を変化させることにより、画像チリ量が異なるサンプルを作成した。尚、画像チリ量を変化させても、画像チリの方向性は変化しない。
【００６８】
第１の実施例同様のジッタに対する画像チリの影響の評価、また、第３の実施例同様に、色重ねの中の１色をスクリーンピッチの半分だけ、それぞれスクリーンの谷間にずれるよう設定した印字パターンにてレジストレーションのずれに対する画像チリの影響を評価した。
【００６９】
画像評価は第１の実施例に基づき、複数の被験者の判定の最多数判定による。表４は、本発明の画像形成装置で横万線スクリーンによる画像判定結果の結果で、それぞれの判定がスクリーンピッチ、画像チリ量によってどう判定されるかを示した。
【００７０】
【表４】

【００７１】
表４中のジッタについての判定は、第１の実施例同様、◎は非常に良好の判定、○は良好の判定、×は不良の判定であり、画像チリ量は、スクリーンピッチによらず、より大きい方がジッタが目立たず良好になる。第１の実施例同様に、画像チリが１３μｍと小さい時にはジッタがはっきりと目立ってしまい画質が低下している不良の判定であり、１５μｍ以上であれば良好の判定であるが、第２の実施例であるドットスクリーンによる画像や第３の実施例である縦万線スクリーンで得た画像より全体に滑らかありで良好な画像が得られ、スクリーンピッチの大きさによらず、ジッタ判定が非常に良好な領域が広範囲であった。よって、横万線スクリーン構造の最終画像に１５μｍ以上の画像チリがあれば、ジッタを目立たせることがない良好な画像を得ることができる。
【００７２】
次に色再現性についての判定は、第３の実施例同様、◎は色再現安定性が完全に確保された非常に良好なサンプル、○は色再現安定性が確保された良好なサンプル、△は色再現安定性がやや悪いが実用上問題はない許容サンプル、×は色再現安定性が悪い不良サンプルである。第３の実施例である縦万線スクリーンで得た画像と比較して、更に広範囲の画像チリ量において安定した色再現性が確保され、縦万線スクリーンではやや色再現性が不安定であったスクリーンピッチが広いサンプルでも、色再現性が安定した。
【００７３】
次にライン感についての判定であるが、表４中のライン感の判定は、第３の実施例同様、◎は非常に良好、○は良好、△は許容の判定である。本実施例の横万線スクリーンのライン感はスクリーンピッチによる差は殆どなく、未印部分を画像チリが選択的に埋め、印字部分には必要以上に画像チリが重複しないために、ハイライト部においてもスクリーンを目立たせることがなく、良好な画像を得ることができ、非常に良好な判定であった画像チリ量の領域が、第３の実施例である縦万線スクリーンより更に広範囲であった。
【００７４】
更に本実施例では、画像チリ量が大きいにも関わらず画像の滲みがないことも改善された。滲みの判定は第２及び第３の実施例同様、◎は非常に明瞭、○は明瞭、×は不明瞭の判定であり、これまでの実施例より更に広い画像チリ量の領域で、非常に明瞭な画像であった。これは、画像チリに方向性があることによりスクリーンの間の未印字部分へ画像チリが選択的に広がっていくために、万線スクリーンが互いに干渉することなく未印字部分を埋めることができるからであり、これにより、細部の表現も可能であり、多彩な階調表現もできる。また、未印字部分を画像チリが選択的に埋め、印字部分には必要以上に画像チリが重複しないために、ハイライト部においてもスクリーンが目立たず、良好な画像を得ることができる。
【００７５】
以上により横万線スクリーンでは、ジッタを目立たせることがなく良好な画像が得られ、また、レジストレーションのずれに対しても常に色再現性安定性を確保することができる。また、ハイライト部においてもスクリーンが目立たず、かつ画像チリ量が大きいにも関わらず画像の滲みがない、幅広い領域の画像チリ量で良好な画像を得ることができる。
【００７６】
また、前述の万線スクリーンは、横万線スクリーンとはならなくても、被記録材の送り方向に角度をつけるだけでもよい。更に万線スクリーンではなくても、ドットスクリーンにおいて被記録材の送り方向に対して角度を持つスクリーンでもよい。
【００７７】
以上の実施例より、画像チリ量が大きくなっても良好な画像が得られるため、画像形成装置での条件設定、特に画像チリ発生を容易にコントロールできる転写工程での条件設定の自由度が高くなり、画像形成装置全体の設計をより自由なものにすることも可能である。
【００７８】
また、本発明の実施例では画像形成装置を乾式トナーを用いる電子写真方式でのみ説明してきたが、液体現像による電子写真方式の画像形成装置や、或いはインク滴で画像形成するインクジェット方式による画像形成装置でもよい。
【００７９】
【発明の効果】
本発明の画像形成装置によれば、最終画像の画像チリ量が所定値以上即ち１５μｍ以上であることによって、濃度変動が画像チリの存在により適度にぼかされているために、ノイズが発生しても人の目は感じにくくなるため、ジッタを目立たせることがない、良好な画像を得ることができる。
【００８０】
また、本発明の画像形成装置は、スクリーンを形成し、かつ、画像チリ量が所定値以上即ち１５μｍ以上でスクリーンピッチの１／２以下であるドットスクリーン構造であることにより、ジッタを目立たせることがなく、良好な画像を得ることができる。また、スクリーンの周囲が適度にぼけるので、ドットスクリーン特有のモアレやロゼッタパターンを目立たせることがなく、ハイライト部でもスクリーンを構成するドットのドット感がない良好な画像を得ることができる。また、ハイライト部においても隣接するドットに過剰に干渉することがないので、画像の滲みがなく明瞭で良好な画像を得ることができる。
【００８１】
また、本発明の画像形成装置は、スクリーンを形成し、かつ、画像チリ量が所定値以上即ち１５μｍ以上でスクリーンピッチの１／２以下である万線スクリーン構造であることにより、レジストレーションのズレに対しても、常に色再現安定性を確保することができる。また、スクリーンの周囲が適度にぼけるので、万線スクリーン特有の、画像中の斜めに伸びる細線や斜めコントラストエッジとスクリーンが交差する時の階段状のぎざぎざを目立たせることがない良好な画像が得られ、ハイライト部でもスクリーンを構成するラインのライン感がない良好な画像を得ることができる。また、ハイライト部においても隣接するラインに過剰に干渉することがないので、画像の滲みがなく明瞭で良好な画像を得ることができる。
【００８２】
また、画像チリ量が所定量以上かつスクリーンピッチの１／２以下の万線スクリーンを、被記録材の送り方向に対して横万線スクリーンにすることによって、ジッタを常に目立たせることがない良好な画像を得ることができる。また、レジストレーションのズレに対しても、常に色再現安定性を確保することができる。また、スクリーンの周囲が適度にぼけるので、万線スクリーン特有の、画像中の斜めに伸びる細線や斜めコントラストエッジとスクリーンが交差する時の階段状のぎざぎざを目立たせることがない良好な画像が得られ、未印字部分を画像チリが選択的に埋め、印字部分には必要以上に画像チリが重複しないために、ハイライト部でも更にスクリーンを構成するラインのライン感がない良好な画像を得ることができる。また、隣接するラインに過剰に干渉することがないので、画像の滲みがなく明瞭で良好な画像を得ることができる。
【図面の簡単な説明】
【図１】本発明の画像形成装置の主要断面図。
【図２】本発明の画像形成装置で用いる画像チリ量について説明する図。
【図３】本発明の画像形成装置のドットスクリーンによる最終画像サンプルの拡大図。
【図４】本発明の画像形成装置の万線スクリーンによる最終画像サンプルの拡大図。
【図５】本発明の画像形成装置における方向性を有する画像チリの一例の拡大図。
【図６】本発明の画像形成装置における横万線スクリーンによる最終画像の拡大図。
【符号の説明】
１０１ 感光体
１０２ 帯電ローラ
１０３ 露光手段
１０４ 折り返しミラー
１０５Ｙ イエロー現像器
１０５Ｍ マゼンタ現像器
１０５Ｃ シアン現像器
１０５Ｋ ブラック現像器
１０６ 中間転写体
１０７ １次転写ローラ
１０８ １次転写用電源
１０９ 感光体クリーナー
１１０ 除電ランプ
１１１ 給紙手段
１１２ 給紙カセット
１１３ 被記録材
１１４ レジストローラ対
１１５ 駆動ローラ
１１６ ２次転写ローラ
１１７ ２次転写用電源
１１８ テンションローラ
１１９ 中間転写体クリーナー
１２０ 定着手段
２０１ 画像チリ量の測定に用いる拡大画像
２０２ 細線
２０３ 画像チリ
２０４ａ 画像チリ測定ライン
２０４ 輝度プロファイル
２０５ ７０％〜１０％点間の距離
２０６ ドット
２０７ ドットスクリーンのスクリーンピッチ
２０８ ライン
２０９ 万線スクリーンのスクリーンピッチ
２１０ 露光主走査方向
２１１ 被記録材の送り方向[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus that processes input image data, forms an image, and obtains a multicolor final image on a recording material. More particularly, the present invention relates to an image forming apparatus that obtains a multicolor final image on a recording material using electrophotography.
[0002]
[Prior art]
Generally, in an apparatus for forming an image using electrophotography, a latent image is formed in an exposure process for input image data, and a toner as a developer moves through a development process and a transfer process. A final image is obtained on the recording material. Due to the movement of the toner, the final image gradually changes with respect to the input image data, and the image quality of the final image is determined.
[0003]
Conventionally, the generated image dust was supposed to appear as image defects such as blurring and blurring of characters and edges of contrast images. Therefore, it is generally said that in order to improve the image quality, it is better that the toner does not scatter during the process of moving the toner. In order to prevent the toner from scattering, the chargeability and fluidity of the toner itself can be improved, A device has been devised in the process involving the movement of, particularly in the transfer process. For example, JP-A-5-289530 discloses that the intermediate transfer medium is improved, and JP-A-4-318578 discloses that the transfer electric field is improved and toner scattering is prevented in order to obtain a good image.
[0004]
[Problems to be solved by the invention]
As described above, in the conventional image forming apparatus, it is preferable that there is no toner scattering (hereinafter referred to as image dust) in order to improve image quality. However, the present inventor has found that image dust of a predetermined value or more is necessary for improving the image quality. For example, in a conventional image forming apparatus that prevents image dust, when jitter appears on the final image due to exposure jitter, drive system jitter, mechanical feed unevenness, etc., it is extremely noticeable as noise. This is a problem because the image quality deteriorates.
[0005]
In addition, in an image forming apparatus that forms a color image by repeating single-color image formation, if the registration of each color image deviates even slightly during color superposition, the color appearance of the final image will change, so that color reproduction is stable. Sexuality gets worse. This is an important problem specific to an image forming apparatus that forms a color image.
[0006]
In some cases, an image is formed with a screen structure as gradation expression, but the screen structure in a halftone image portion, particularly a highlight portion having a very low density, is conspicuous, which is a problem.
[0007]
According to the present invention, the presence of image dust appearing in the entire image area is important in order to improve the image quality without conspicuous such jitter, registration deviation, screen structure, etc. Based on the new knowledge of the present inventors, the objective is to make the image quality with good color reproducibility without conspicuous jitter, and the screen structure even in a halftone image. An object of the present invention is to provide a color image forming apparatus in which the amount of image dust is not less than a predetermined range in order to obtain an inconspicuous and smooth image.
[0008]
[Means for Solving the Problems]
In view of the above, The present invention Processes input image data, forms an image, and places it on the recording material Multicolor screen In the image forming apparatus for obtaining the final image, the final image has an image dust amount of 15 μm or more. And less than 1/2 the screen pitch It is characterized by being.
[0012]
further The present invention The screen is a dot screen.
[0013]
further The present invention The screen is a line screen.
[0014]
further The present invention The line screen is a set of lines perpendicular to the feeding direction of the recording material.
that's all
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described based on examples and drawings. First, the image forming apparatus of the present invention will be briefly described with reference to FIG.
[0016]
FIG. 1 is an example of an image forming apparatus of the present invention, and is a main cross-sectional view of an electrophotographic image forming apparatus. The charging roller 102 uniformly charges the photoconductor 101. The laser beam formed by the exposure means 103 in accordance with the input image data is guided onto the photoconductor 101 by the folding mirror 104 to form an electrostatic latent image. Next, of the one-component contact type developing device 105 that can be contacted / separated in the direction of the arrow in FIG. The toner moves, reversely develops, and is visualized on the photosensitive member 101. The visualized yellow toner is transferred onto the intermediate transfer member 106 by a primary transfer power source 108 applied to the primary transfer roller 107 with a reverse polarity to the toner, and transferred and moved by the primary transfer electric field effect. To do. Further, the intermediate transfer member 106 is adjusted by a tension roller 118 so that an appropriate tension is applied. The untransferred toner is collected by a photoconductor cleaner 109 that is cleaned by bringing the blade into contact therewith, and then the photoconductor potential is reset by the charge eliminating lamp 110. The same operation is synchronized with the position of the intermediate transfer member 106 and the light emission timing of the exposure unit 103, and is repeated for the magenta developing unit 105M, the cyan developing unit 105C, and the black developing unit 105K. The toner is superimposed to form a full color image. The secondary transfer roller 116 and the intermediate transfer body cleaner 119 are in a separated state. On the other hand, the recording material 113 is conveyed from the paper feeding cassette 112 to the registration roller pair 114 by the paper feeding means 111 and then synchronized with the full color image on the intermediate transfer member 106 in the direction of the arrow in the drawing. It is conveyed to a secondary transfer portion formed by a secondary transfer roller 116 that can be contacted and separated. In the secondary transfer portion, the secondary transfer roller 116 contacts the intermediate transfer member 106 in synchronization with the recording material 113 to form and press the nip portion and calculate the voltage obtained from the primary transfer power source 108. The voltage determined in 121 is controlled at a constant voltage by the secondary transfer power source 117, and the toner layer moves on the recording material 113 by the electric field action, thereby forming a full-color toner image. Thereafter, the recording material 113 is fixed by the fixing unit 120 and discharged out of the apparatus.
[0017]
Thus, until the final image is obtained, the toner is moved a plurality of times, and is input based on the properties of the toner itself such as toner fluidity, charge amount, particle shape and particle size, or electric field action and mechanical action. An image dust amount is generated on the recording material 113 for the obtained image data.
[0018]
In the image forming apparatus shown in FIG. 1, the properties of toner such as particle size, charge amount, and fluidity, and process conditions such as charging, exposure, development, transfer, and fixing are appropriately selected and adjusted so that recording is possible. An image dust amount of a predetermined value or more is generated and output as a final image on the material 113. Particularly in the transfer process, it is easy to control the movement of the toner, and the state of image dust can be changed by adjusting the transfer electric field. In the image forming apparatus of FIG. 1, in the primary transfer in which the toner moves from the photosensitive member 101 to the intermediate transfer member 106, the constant current value setting value is about 5 to 20 μA, and recording is performed from the intermediate transfer member 106. In the secondary transfer in which the toner moves onto the material 113, the constant current set value is 10 to 30 μA. The larger the constant current set value is, the more the image dust is generated.
[0019]
In addition to the transfer electric field, image dust can be easily controlled by using toners having different fluidity. There are two types of toner: a toner by polymerization and a toner by pulverization. In both toners, the fluidity can be freely changed by appropriately selecting the type and amount of the fluidity modifier externally added to the toner. Image dust is more likely to occur as the toner does not flow.
[0020]
As described above, the size of image dust can be freely controlled by combining the transfer electric field and the fluidity of the toner.
[0021]
Here, the amount of image dust defined in the present invention will be described with reference to FIG. FIG. 2 is a diagram illustrating the amount of image dust used in the image forming apparatus of the present invention. The enlarged image 201 used for measuring the amount of image dust is, for example, a collection of a plurality of thin lines 202 spaced apart, and an image dust 203 spreads around the thin lines 202. The enlarged image 201 is captured by a CCD camera, and a brightness profile 204 is obtained for an image dust measurement line 204a that is perpendicular to the direction in which the thin lines 202 are aligned. If the peak value (low luminance) of the image portion of the thin line 202 of the luminance profile 204 is 100% luminance, and the peak value (high luminance) of the image portion that is the gap in which the thin lines 202 are arranged is 0% luminance, the luminance profile 204 A plurality of distances 205 between 70% to 10% points are measured, and the average value is used as the image dust amount. The image used for the measurement is suitably a set of fine lines or dots arranged at intervals of 80 μm to 2 mm, and even if an image with such a print pattern is formed or an image with a screen structure, the lines or dots constituting the screen. May be used as they are.
[0022]
Hereinafter, the present invention will be described in detail with reference to examples.
[0023]
[Example 1]
In the image forming apparatus of the present invention, when exposure, development, transfer, and fixing processes are performed in a line sequential manner as shown in FIG. 1, jitter is likely to occur in each process. Jitter is generated due to the exposure of the exposure device 103 due to the drive of the photosensitive member 101, the intermediate transfer member 106, and the like, and speed fluctuations and vibrations in the development, transfer, and fixing processes. In order to prevent this, it is necessary to apply grease to the gears, but it is difficult to completely eliminate jitter on the drive transmission system.
[0024]
Therefore, in the image forming apparatus of the present invention, in the first embodiment, in the electrophotographic process section of FIG. 1, the constant current value set value is set to 15 μA in the primary transfer, and the constant current set value in the secondary transfer. Was adjusted between 10 and 30 μA. The toner used was a pulverized toner having a volume average particle diameter of 7 μm and a polymerized toner having a volume average particle diameter of 7 μm. The fluidity of the toner is also adjusted by the amount and type of the fluidity modifier externally added to the toner particles, and a toner having the desired fluidity can be obtained. By combining this with a transfer electric field, it is possible to obtain samples with the desired image dust amount, which are designated as samples A, B, C, and D. The fluidity of the toner is expressed as a loose apparent density (g / cc), and the larger the value, the easier it is to flow.
[0025]
In addition, one of the gear train gears that transmits the drive to the photosensitive member 101 in order to create a monochrome solid image print pattern on a high-quality copy paper and intentionally generate known periodic jitter, A tooth with a missing part was used.
[0026]
Using the above samples, image evaluation of how the amount of image dust affects the jitter was performed.
[0027]
Image evaluation is based on visual observation of a plurality of subjects. Each subject observes individually, and first judges whether or not the jitter is conspicuous and good, and after the aggregation, the judgment is the most numerous and the final judgment is made.
[0028]
Table 1 shows the relationship between the image dust amount measured based on the above definition and the image quality evaluation. Note that the amount of image dust obtained from the adjustment of toner fluidity and secondary transfer current in Table 1 is unique to the image forming apparatus of this embodiment, and is not limited to this in other image forming apparatuses.
[0029]
[Table 1]

[0030]
In the evaluation of Table 1, ◎ is a very good judgment, jitter is not noticeable at all and the image quality does not deteriorate due to jitter, ○ is a good judgment, and jitter is conspicuous when observed carefully, but does not cause a reduction in image quality A sample, x, is a sample for judging a defect, and the jitter is clearly conspicuous and the image quality is lowered.
[0031]
From Table 1, it can be seen that when the amount of image dust increases in the order of sample D, sample C, sample B, and sample A, the jitter becomes conspicuous and a good image is obtained. This is because the density fluctuation due to jitter is moderately blurred due to the presence of image dust, making it difficult for human eyes to feel.
[0032]
As described above, when the amount of image dust is a predetermined amount, that is, 15 μm or more, a good image can be obtained without conspicuous jitter.
[0033]
[Example 2]
The image forming apparatus of the present invention can also be used for an image with a screen structure. A full-color image often obtains a pictorial image such as a photograph or illustration. At this time, since various gradation expressions are required, an image is often formed with some screen structure. Here, the case where image formation is performed with the screen structure in the image forming apparatus of the present invention will be described.
[0034]
Screen structures include dot screens, line screens, cross-line screens, concentric screens, sand screens, and hybrid screens that combine these. Dot screens and line screens are commonly used for electrophotography. Or a hybrid screen of both. All screen structures are indispensable for reproducing multi-tone pictorial images such as photographs and illustrations. In particular, in the halftone image area, the screen structure allows for gradation and color reproducibility. Since it changes greatly, it is an important factor that determines the quality of the final image.
[0035]
In the second embodiment of the present invention, an image having a dot screen structure is used. Here, a dot screen will be described with reference to FIG. FIG. 3 is an enlarged view of a final image sample by a dot screen in the image forming apparatus of the present invention. The dots 206 constituting the image are regularly aligned with the image dust 203 around the periphery, and the distance between the dot centers with respect to the direction in which the dots 206 are aligned is the screen pitch 207 of the dot screen. In the image forming apparatus of the present invention, a dot screen having a screen pitch of 127 μm with a resolution equivalent to 200 dpi is usually used, but the screen pitch may be changed according to the image density and the type of print sample. The image density is determined by the size of the dots, and gradation expression is thereby performed. In addition, when multiple color dots overlap in full color printing, a screen angle may be provided on the dot screen to prevent moiré caused by overlapping of each color dot.
[0036]
In this example, the screen pitch was set to 127 μm corresponding to 200 dpi, 170 μm corresponding to 150 dpi, and 254 μm corresponding to 100 dpi. As in the first example, samples with different image dust amounts were prepared by changing the transfer electric field and toner fluidity, and the images were evaluated.
[0037]
As in the first embodiment, one of the gears in the train wheel system that transmits the drive to the photoconductor 101 is a tooth in order to intentionally generate a known periodic jitter in order to evaluate the influence of image dust on the jitter. A pattern with a part of the chip was used, and vibration was periodically applied. The print pattern at this time was a 50% density uniform pattern created by a dot screen.
[0038]
In photographic images with a wide range of tones, the effect of image dust on the dot appearance, which is the noticeability of the screen in the halftone image area, especially in the highlight area, and by increasing the image dust amount. We evaluated how it affects the bleeding of images and the details of photographic images.
[0039]
The evaluation is based on the maximum number of determinations of a plurality of subjects based on the first embodiment. Table 2 shows the results of image determination using a dot screen in the image forming apparatus according to the present invention, and how each determination is determined based on the screen pitch and the amount of image dust.
[0040]
[Table 2]

[0041]
In Table 2, the evaluation of jitter is the same as in the first embodiment, ◎ is a very good judgment, jitter is not conspicuous and the image quality does not deteriorate due to jitter, ○ is a good judgment, and the jitter is careful A sample that is conspicuous when observed but does not lead to a decrease in image quality, and X is a sample that is judged to be defective, and the jitter is clearly conspicuous to deteriorate the image quality. The larger the image dust amount is, the higher the jitter becomes inconspicuous, the better the determination, regardless of the screen pitch. As in the first embodiment, when the image dust is as small as 13 μm, the jitter is clearly noticeable and the image quality is deteriorated. It is a good judgment that does not reach. Therefore, if there is an image dust of 15 μm or more in the final image of the dot screen structure, a good image that is not noticeable can be obtained even if there is jitter. This is because, on a screen without image dust, if jitter appears in a periodic screen recognized as a signal, that is, a dot, it is recognized as noise, which increases the sensitivity of the human eye, causing discomfort, and lowering the image quality. However, if the density fluctuation around the dot is moderately blurred due to the presence of image dust, the recognition as a signal becomes ambiguous, and even if noise occurs, it is difficult for the human eye to feel it. .
[0042]
Next, regarding the determination of the dot feeling constituting the screen evaluated in the photographic image, the determination of the dot feeling in Table 2 is as follows. Then, a good sample with a feeling of dots, and Δ is an acceptable sample with a feeling of dots but no practical problems.
[0043]
The dot feeling, which is the conspicuousness of the dot screen, also depends on the screen pitch. The smaller the screen pitch, the better the dot feeling disappears. The larger the image dust at any screen pitch, the better the dot feeling. Sample. The screen pitch of 127 μm (equivalent to 200 dpi) used as a standard in the image forming apparatus of the present invention is a good sample even when the image dust amount is as small as 13 μm. The larger the image dust amount, the more the dot feeling disappears. In addition, moire that occurs despite the provision of a screen angle and a rosette pattern unique to a dot screen are not conspicuous, and a good image can be obtained.
[0044]
On the other hand, if the amount of image dust is too large, the entire image may blur and an unclear image may be formed. This phenomenon is more conspicuous in the halftone image portion where the color and shape of the image are more easily recognized than in the shadow portion where the whole is dark. Judgment in Table 2 shows that ◎ is a very clear judgment, and there is no blur at all, and the edge is very clearly expressed. ○ is a clear judgment. A sample, x is an unclear determination, and is a sample that is greatly blurred and expressed with blurred edges. The relationship between the amount of image dust and the clarity of the image depends on the screen pitch. When the screen pitch is large, even if the amount of image dust is large, the image does not matter in terms of bleeding, and a good image is obtained. As long as the image dust does not exceed 1/2 of the screen pitch, a clear and good image can be obtained without excessive interference with adjacent dots even in the halftone image portion.
[0045]
Thus, in an image having a dot screen structure, when the image dust amount is a predetermined amount, that is, 15 μm or more, jitter does not stand out, and a good image can be obtained. In addition, it is possible to obtain a good image without the dot feeling of the dots constituting the screen. In addition, when the image dust amount does not exceed 1/2 of the screen pitch, a clear and good image can be obtained without blurring of the image.
[0046]
Example 3
In the third embodiment, a final image with a line screen structure in the image forming apparatus of the present invention will be described. First, the line screen will be described with reference to FIG. FIG. 4 is an enlarged view of a final image sample by a line screen in the image forming apparatus of the present invention. The lines 208 constituting the image are regularly aligned so that one line perpendicular to the exposure main scanning direction 210 is sequentially formed with the image dust 203 in the periphery, and the distance between the centers of the lines is a line screen. Screen pitch 209. In the image forming apparatus of the present invention, a line screen having a screen pitch of 127 μm corresponding to a resolution of 200 lpi is usually used, but the screen pitch may be changed according to the image density and the type of print sample. In the line screen, the density is determined by the thickness of the line, and gradation expression is thereby performed.
[0047]
Further, in color superposition with a multicolor line screen in full-color printing, the line 208 overlaps at the same position, is thermally melted in the fixing unit 120, and color is developed based on subtractive color mixture. However, when the line screen is slightly shifted from the target position, the color appearance may be different, that is, the color reproduction stability may be deteriorated. Such registration misalignment may occur in the electrophotographic process unit of FIG. 1 due to positional accuracy in the exposure process, vibration due to the drive system of the photosensitive member 101 or the intermediate transfer member 106, speed fluctuation due to an accuracy error, or the like. is there.
[0048]
In this embodiment, the screen pitch is set to 127 μm corresponding to 200 lpi, 170 μm corresponding to 150 lpi, and 254 μm corresponding to 100 lpi. As in the first example, samples with different image dust amounts were prepared by changing the transfer electric field and toner fluidity, and the images were evaluated.
[0049]
In order to evaluate the effect of image dust 203 on registration shifts on line screens, multi-color print patterns created by deliberately shifting registration with respect to normal color overlay are also prepared, and color reproduction stability with both It was evaluated how it affected. The print pattern at this time is a 50% density uniform pattern created on a line screen, and a pattern in which lines 208 of normal colors are layered and overlapped at the same position, and an image in which registration is shifted with respect to this pattern Thus, evaluation was performed with a set print pattern so that one color in the color overlap was shifted in the exposure main scanning direction 210 by half the screen pitch (shifted in the valley of the screen). For photographic images with a wide range of gradations, it was evaluated whether the line feeling of the line screen structure was particularly noticeable in the highlight area, and how the image blur was affected by increasing the amount of image dust.
[0050]
The image evaluation is based on the determination of the largest number of determinations of a plurality of subjects based on the first embodiment. Table 3 shows image determination results using a line screen in the image forming apparatus of the present invention, and shows how each determination is determined by the screen pitch and the amount of image dust.
[0051]
[Table 3]

[0052]
In Table 3, the color reproduction stability is judged as follows: ◎ is a very good sample in which color reproduction stability is completely ensured, ○ is a good sample in which color reproduction stability is ensured, and △ is color reproduction stability Is an acceptable sample that is secured slightly but has no practical problems, and x is a defective sample with poor color reproduction stability. As the amount of image dust decreases, the color appearance changes between an image sample shifted in registration and a normal one, that is, the color reproduction stability deteriorates.
[0053]
This is because the color mixing methods of the two are different. In a normal screen in which colors are superimposed at the same position, a plurality of color layers are mixed at the same position by the fixing unit 120, and the spectral reflectance of each toner layer can be represented by a curve obtained by substantially integrating each wavelength. Based on the color reproduction. On the other hand, in a screen with a misregistration, the screens are juxtaposed without overlapping, and color reproduction is performed by mixing reflected light from the screens, that is, juxtaposed color mixing which is a kind of additive color mixing. In subtractive color mixing, the color becomes darker than the original color, and the color obtained by juxtaposed color mixing has a lightness almost halfway between the original colors, so that even if the same amount of toner is present on the recording paper, the appearance is different. That is, the color reproduction stability is deteriorated.
[0054]
However, in an image with a large amount of image dust, subtractive color mixing and side-by-side color mixing are mixed in both the color overlap at the same position and the color shift in which the registration is shifted. In this area, subtractive color mixing is performed, so that the color reproduction stability is always ensured even when the color overlap position is changed.
[0055]
With a screen pitch of 127 μm equivalent to 200 lpi that is used as a standard in the image forming apparatus of the present invention, if the image dust amount is 15 μm or more, there is no practical problem in color reproduction stability, and the larger the image dust amount, the better the color reproducibility. Become good. Similarly, when the screen pitch is large and the image dust amount is 15 μm or more, the color reproduction stability is acceptable without any practical problem, but the smaller the screen pitch, the better the color reproduction stability.
[0056]
The above phenomenon related to color superposition is also observed in dot screens. However, when a screen angle is provided in the dot screen, the colors due to juxtaposed color mixing and subtractive color mixing are inherently mixed, and the amount of image dust Even if the registration of the sample is small, the line screen will not appear as a significant difference in color reproducibility.
[0057]
Also, due to transferability problems when multiple layers of toner are stacked, if the image dust is small, each color toner becomes a layer and the transfer performance varies depending on the distance from the transfer medium, which greatly varies the state of the image dust, Since the area ratio differs between a layer without image dust and a layer to which image dust is transferred over a wide range, color reproduction stability cannot be ensured. For example, at the time of secondary transfer transferred from the intermediate transfer body 106 to the recording material 113, the amount of image dust is small in the toner layer closest to the recording material 113 and is transferred over a wide range as the distance from the recording material increases. The color of the toner layer farthest from the recording material is close to the hue. On the other hand, when the amount of image dust is originally large, the state of image dust does not change so much in each color toner layer, and color reproduction stability is always ensured.
[0058]
Next, regarding the feeling of lines constituting the screen evaluated in photographic images, the feeling of lines in Table 3 is as follows: ◎ is a very good sample with no line feeling, ○ is carefully observed Then, it is a good sample with a line feeling, and Δ is an acceptable sample with a line feeling but no problem in practical use. The line feeling, which is the conspicuousness of the screen, largely depends on the screen pitch as in the case of the dot screen. However, the larger the image dust at any screen pitch, the better the sample without line feeling. The screen pitch of 127 μm (equivalent to 200 dpi) used as a standard in the image forming apparatus of the present invention is a good sample even when the image dust amount is as small as 13 μm. The larger the image dust amount, the more the line feeling disappears. However, this is a problem inherent to the screen image of a line, and thin lines extending in an oblique direction with respect to the lines constituting the screen and a step-like jaggedness caused by the screen pitch are conspicuous in the portion where the edge of the contrast image is present obliquely. Therefore, a good image can be obtained.
[0059]
On the other hand, like the dot screen of the second embodiment, if the amount of image dust becomes too large, the entire image may blur and an unclear image may be formed. Judgment in Table 3 shows that ◎ is a very clear judgment, a sample that has no blur and the edges are very clearly expressed, and ○ is a clear judgment that is slightly blurred but the edges are sufficiently expressed. A sample, x is an unclear determination, and is a sample that is greatly blurred and expressed with blurred edges. The relationship between the amount of image dust and the clarity of the image depends on the screen pitch. When the screen pitch is large, even if the amount of image dust is large, the image does not matter in terms of bleeding, and a good image is obtained. As long as the image dust does not exceed 1/2 of the screen pitch, a clear and good image can be obtained without excessive interference with adjacent lines even in the halftone image portion.
[0060]
Although not shown, the effect of image dust on the jitter was evaluated as in the second embodiment, but the result is that the jitter is conspicuous if the amount of image dust is 15 μm or more, as in the second embodiment. Therefore, a good image can be obtained, and the determination does not change even if the screen pitch is changed.
[0061]
As described above, in an image having a line screen structure, color reproduction stability can always be ensured when the image dust amount is a predetermined amount, that is, 15 μm or more. In addition, it is possible to obtain a good image without a feeling of lines constituting the screen even in the highlight portion. In addition, when the image dust amount does not exceed 1/2 of the screen pitch, a clear and good image can be obtained without blurring of the image.
[0062]
Example 4
Image dust generated in the image forming apparatus may not only be isotropically spread as shown by the dot screen of FIG. 3, for example, but also spread with directionality. In particular, in the process of forming an image in a line sequential manner, it is affected by driving of the photosensitive member 101 and the intermediate transfer member 106, or due to some cause such as speed fluctuation or vibration in each process of development, transfer, and fixing, In many cases, image dust has a directionality unique to each image forming apparatus. The image forming apparatus of the present invention can also be used in the case where the image dust has directionality as described above.
[0063]
Here, the direction of the image dust will be described with reference to FIG. 5, taking a dot screen structure as an example. FIG. 5 is an enlarged view of an example of image dust having directionality in the image forming apparatus of the present invention. The dots 206 are aligned with an image dust 203 around the screen at an interval of a screen pitch 207 and at an angle with respect to the recording material feed direction 211. The image dust 203 is fed in the recording material feed direction of the image forming apparatus. It extends so as to be parallel to the direction of 211. In general, the image forming apparatus tends to be scattered in the recording material feeding direction 211. When the image dust spreads with directionality, for example, in the case of a dot screen as shown in FIG. 5, the direction in which the dots are aligned (the direction in which the dots are close to each other) and the direction in which the dots are spread have different angles. The image dust 203 is further spread to the unprinted portion.
[0064]
Therefore, in the fourth embodiment of the present invention, as shown in FIG. 6, the above-mentioned line screen is a horizontal line screen that forms a single line 208 perpendicular to the recording material feed direction 211. An image was formed. FIG. 6 is an enlarged view of the final image by the horizontal line screen in the image forming apparatus of the present invention. In a horizontal line screen with a screen pitch of 209, when the image dust 203 spreads in the recording material feed direction 211, the image forming line 208 spreads uniformly between the screens that are not printed. It has Chile 203.
[0065]
The image forming apparatus used in this embodiment has a unique image dust direction. The direction of image dust extends long in the feed direction of the recording material, and when a perfect dot is printed, the feeding direction of the recording material becomes an ellipse whose major axis is the major axis, and the ratio of the major axis to the minor axis is almost The image dust becomes 1: 0.7.
[0066]
In this embodiment, the screen pitch is set to 127 μm corresponding to 200 lpi, 170 μm corresponding to 150 dpi, and 254 μm corresponding to 100 dpi. As in the first example, samples with different image dust amounts were prepared by changing the transfer electric field and toner fluidity, and the images were evaluated.
[0067]
The print pattern was a uniform pattern of 50% density created with a dot screen, and samples with different image dust amounts were created by changing the transfer electric field and toner fluidity, as in the first example. Note that the direction of the image dust does not change even if the amount of image dust is changed.
[0068]
Evaluation of the effect of image dust on the jitter as in the first embodiment, and printing in which one color in the color overlap is set to be shifted by half the screen pitch, respectively, in the valley of the screen, as in the third embodiment. The effect of image dust on registration shift was evaluated by pattern.
[0069]
The image evaluation is based on the determination of the largest number of determinations of a plurality of subjects based on the first embodiment. Table 4 shows the results of image determination using a horizontal line screen in the image forming apparatus of the present invention, and how each determination is determined based on the screen pitch and the image dust amount.
[0070]
[Table 4]

[0071]
Judgment on jitter in Table 4 is the same as in the first embodiment, ◎ is very good judgment, ◯ is good judgment, x is bad judgment, and the image dust amount is independent of the screen pitch. Larger values make the jitter less noticeable and better. As in the first embodiment, when the image dust is as small as 13 μm, the jitter is clearly noticeable and the image quality is deteriorated. When the image dust is 15 μm or more, the determination is good. A smooth and good image can be obtained as a whole than the image obtained by the dot screen as an example and the image obtained by the vertical line screen as the third embodiment, and the jitter judgment is very high regardless of the size of the screen pitch. The good area was extensive. Therefore, if there is an image dust of 15 μm or more in the final image of the horizontal line screen structure, it is possible to obtain a good image without conspicuous jitter.
[0072]
Next, as for the color reproducibility, as in the third embodiment, ◎ is a very good sample in which color reproduction stability is completely ensured, ○ is a good sample in which color reproduction stability is ensured, Δ Is an acceptable sample with slightly poor color reproduction stability but no problem in practical use, and x is a defective sample with poor color reproduction stability. Compared with the image obtained with the vertical line screen of the third embodiment, stable color reproducibility is secured in a wider range of image dust, and the color reproducibility is somewhat unstable with the vertical line screen. The color reproducibility was stable even for samples with a wide screen pitch.
[0073]
Next, regarding the line feeling, in the determination of the line feeling in Table 4, as in the third embodiment, ◎ is very good, ◯ is good, and Δ is acceptable. The line feeling of the horizontal line screen of this embodiment has almost no difference depending on the screen pitch, and the image dust is selectively filled in the unmarked portion and the image dust does not overlap more than necessary in the printed portion. In this case, a good image can be obtained without making the screen stand out, and the area of the image dust amount, which was a very good judgment, is wider than the vertical line screen of the third embodiment. It was.
[0074]
Furthermore, in this example, it was also improved that there was no blurring of the image despite the large amount of image dust. As in the second and third embodiments, the blur is judged as very clear, ◯ is clear, and x is unclear, and the image dust is much wider than in the previous embodiments. It was a clear image. This is because the image dust selectively spreads to the unprinted portion between the screens due to the direction of the image dust, so that the line screen can fill the unprinted portion without interfering with each other. Thus, it is possible to express details and to express various gradations. In addition, since the image dust is selectively filled in the unprinted portion and the image dust does not overlap more than necessary, the screen is not conspicuous even in the highlight portion, and a good image can be obtained.
[0075]
As described above, with the horizontal line screen, it is possible to obtain a good image without conspicuous jitter, and it is possible to always ensure the stability of color reproducibility even with respect to a registration shift. Further, even in the highlight portion, a good image can be obtained with a wide range of image dust amounts in which the screen is not conspicuous and there is no image blur despite the large amount of image dust.
[0076]
In addition, the above-described line screen may not be a horizontal line screen, but may be provided with an angle in the recording material feeding direction. Furthermore, a screen having an angle with respect to the recording material feeding direction in the dot screen may be used instead of the line screen.
[0077]
From the above embodiments, a good image can be obtained even when the amount of image dust is large, so that the degree of freedom in setting conditions in the image forming apparatus, particularly in the transfer process that can easily control the generation of image dust, is high. Thus, the design of the entire image forming apparatus can be made more free.
[0078]
In the embodiments of the present invention, the image forming apparatus has been described only with an electrophotographic system using dry toner. However, an electrophotographic image forming apparatus using liquid development, or an image forming apparatus using an ink jet system that forms an image with ink droplets. It may be a device.
[0079]
【The invention's effect】
According to the image forming apparatus of the present invention, when the image dust amount of the final image is equal to or greater than a predetermined value, that is, 15 μm or more, the density fluctuation is moderately blurred due to the presence of image dust, and thus noise is generated. However, since it is difficult for human eyes to perceive, it is possible to obtain a good image without making jitter noticeable.
[0080]
Further, the image forming apparatus of the present invention forms a screen and has a dot screen structure in which the amount of image dust is a predetermined value or more, that is, 15 μm or more and 1/2 or less of the screen pitch, so that jitter is conspicuous. And a good image can be obtained. Further, since the periphery of the screen is moderately blurred, a moire or rosette pattern peculiar to the dot screen is not conspicuous, and a good image without the dot feeling of the dots constituting the screen can be obtained even in the highlight portion. Further, since there is no excessive interference with adjacent dots even in the highlight portion, a clear and good image can be obtained without blurring of the image.
[0081]
Further, the image forming apparatus of the present invention forms a screen and has a line screen structure in which the image dust amount is not less than a predetermined value, that is, not less than 15 μm and not more than 1/2 of the screen pitch. However, the color reproduction stability can always be ensured. In addition, since the periphery of the screen is moderately blurred, it is possible to obtain a good image that does not stand out from the slanted lines in the image and the stair-like jaggedness when the screen intersects the diagonal contrast edge. As a result, it is possible to obtain a good image without a feeling of lines constituting the screen even in the highlight portion. Further, since there is no excessive interference with adjacent lines even in the highlight portion, a clear and good image can be obtained without blurring of the image.
[0082]
In addition, a line screen with an image dust amount of a predetermined amount or more and 1/2 or less of the screen pitch is made into a horizontal line screen with respect to the feeding direction of the recording material so that jitter is not always noticeable. Can be obtained. Also, it is possible to always ensure color reproduction stability against registration shift. In addition, since the periphery of the screen is moderately blurred, it is possible to obtain a good image that does not stand out from the slanted lines in the image and the stair-like jaggedness when the screen intersects the diagonal contrast edge. Since the image dust is selectively filled in the unprinted portion and the image dust does not overlap more than necessary in the printed portion, it is possible to obtain a good image without the feeling of lines constituting the screen even in the highlight portion. Can do. Further, since there is no excessive interference with adjacent lines, a clear and good image can be obtained without blurring of the image.
[Brief description of the drawings]
FIG. 1 is a main cross-sectional view of an image forming apparatus of the present invention.
FIG. 2 is a diagram illustrating an image dust amount used in the image forming apparatus of the present invention.
FIG. 3 is an enlarged view of a final image sample by a dot screen of the image forming apparatus of the present invention.
FIG. 4 is an enlarged view of a final image sample by a line screen of the image forming apparatus of the present invention.
FIG. 5 is an enlarged view of an example of image dust having directionality in the image forming apparatus of the present invention.
FIG. 6 is an enlarged view of a final image by a horizontal line screen in the image forming apparatus of the present invention.
[Explanation of symbols]
101 photoconductor
102 Charging roller
103 Exposure means
104 Folding mirror
105Y Yellow developer
105M Magenta developer
105C cyan developer
105K black developer
106 Intermediate transfer member
107 Primary transfer roller
108 Power supply for primary transfer
109 Photoconductor cleaner
110 Static elimination lamp
111 Paper feeding means
112 Paper cassette
113 Recording material
114 Registration roller pair
115 Drive roller
116 Secondary transfer roller
117 Power supply for secondary transfer
118 Tension roller
119 Intermediate transfer cleaner
120 fixing means
201 Enlarged image used to measure the amount of image dust
202 Fine wire
203 images Chile
204a Image Chile Measurement Line
204 brightness profile
205 Distance between 70% and 10% points
206 dots
207 dot screen pitch
208 lines
Screen pitch of 209,000 line screen
210 Exposure main scanning direction
211 Feed direction of recording material

Claims (4)

In an image forming apparatus that processes input image data, forms an image, and obtains a final image of a multi-colored screen on a recording material, the final image has an image dust amount of 15 μm or more and a screen pitch. An image forming apparatus characterized in that it is 1/2 or less . The image forming apparatus according to claim 1, wherein the screen is a dot screen. The image forming apparatus according to claim 1, wherein the screen is a line screen. The image forming apparatus according to claim 3 , wherein the line screen is a set of lines perpendicular to a feeding direction of the recording material.

Images