JP3969004B2 - Image forming apparatus and image forming method - Google Patents

Description

【００２７】
図３は、第１実施形態における濃度調整因子の最適化処理を示すフローチャートである。この第１実施形態では、まず、パッチセンシング処理によって、高濃度側目標濃度（ＯＤ＝１．２）でトナー像を形成するための露光エネルギーＥと現像バイアスＶbとの相関データを「通常用高濃度側相関データ」として、また高濃度側目標濃度（ＯＤ＝０．８）でトナー像を形成するための露光エネルギーＥと現像バイアスＶbとの相関データを「セーブ用高濃度側相関データ」として求める（ステップＳ２１）。より具体的には、図４に示すフローチャートで示す手順で制御ユニット１が装置各部を制御し、高濃度側相関データを求めている。
【００２８】
図４は第１実施形態における高濃度側相関データの導出手順を示すフローチャートである。まず、パッチ画像を作成する色を最初の色、例えばブラックに設定する（ステップＳ２１１）。そして、帯電バイアスを予めメモリ１２７に記憶されている初期値に設定する一方、複数のパッチ作成条件を設定する（ステップＳ２１２）。ここでは、露光エネルギーＥをＥ1，Ｅ2，…，Ｅmに変化させるとともに、現像バイアスＶbをＶb(1)，Ｖb(2)，…，Ｖb(n)に変化させている。
【００２９】
このようなパッチ作成条件でベタパッチ画像（高濃度用パッチ画像）を感光体２上に順次形成しながら、各パッチ画像を中間転写ベルト７１の外周面に一次転写する（ステップＳ２１３）。なお、この実施形態では、ベタパッチ画像を形成しているが、ベタパッチ画像に限定されるものではなく、ベタパッチ画像に近い高濃度画像、例えばそのパッチ画像全体に対するドットの面積率が約８０％以上の画像を形成してもよい。
【００３０】
次のステップＳ２１４では、すべてのパッチ作成色についてパッチ画像を作成したか否かを判断し、「ＮＯ」と判断される間は、パッチ作成色を次の色に設定し（ステップＳ２１５）、ステップＳ２１２，Ｓ２１３を繰り返して他のトナー色、つまりシアン（Ｃ）、マゼンタ（Ｍ）、イエロー（Ｙ）のベタパッチ画像を中間転写ベルト７１の外周面上にさらに形成していく。
【００３１】
一方、ステップＳ２１４で「ＹＥＳ」と判断すると、各ベタパッチ画像の光学濃度をパッチセンサＰＳで測定する（ステップＳ２１６）。
【００３２】
これに続いて、ステップＳ２１７で基準目標濃度の高濃度側目標濃度（ＯＤ＝１．２）と一致するパッチ作成条件を「通常用高濃度側相関データ」として抽出する。例えば図５（ａ）に示すように、各パッチ作成条件（Ｅ，Ｖb）で作成されたベタパッチ画像のうち、パッチ作成条件（Ｅ1，Ｖb(2)）、（Ｅ2，Ｖb(3)）、…などで作成されたベタパッチ画像が高濃度側目標濃度と一致する場合、これら（Ｅ1，Ｖb(2)）、（Ｅ2，Ｖb(3)）、…が通常用高濃度側相関データとなる。
【００３３】
また、ステップＳ２１８でトナーセーブ用目標濃度の高濃度側目標濃度（ＯＤ＝０．８）と一致するパッチ作成条件を「セーブ用高濃度側相関データ」として抽出する。例えば図６（ａ）に示すように、各パッチ作成条件（Ｅ，Ｖb）で作成されたベタパッチ画像のうち、パッチ作成条件（Ｅ1，Ｖb(4)）、（Ｅm，Ｖb(n)）、…などで作成されたベタパッチ画像が高濃度側目標濃度と一致する場合、これら（Ｅ1，Ｖb(4)）、（Ｅm，Ｖb(n)）、…がセーブ用高濃度側相関データとなる。
【００３４】
このようにして通常用およびセーブ用高濃度側相関データを求めた（ステップＳ２１）後、パッチセンシングによって低濃度側目標濃度（ＯＤ＝０．２）でトナー像を形成するための露光エネルギーＥと現像バイアスＶbとの相関データを低濃度側相関データとして求める（ステップＳ２２）。より具体的には、図７に示すフローチャートで示す手順で制御ユニット１が装置各部を制御し、低濃度側相関データを求めている。
【００３５】
図７は第１実施形態における低濃度側相関データの導出手順を示すフローチャートである。まず、パッチ画像を作成する色を最初の色、例えばブラック（Ｋ）に設定する（ステップＳ２２１）。そして、帯電バイアスを初期値に設定する一方、複数のパッチ作成条件を設定する（ステップＳ２２２）。ここでは、高濃度側相関データを導出する場合と同様に、露光エネルギーＥをＥ1，Ｅ2，…，Ｅmに変化させるとともに、現像バイアスＶbをＶb(1)，Ｖb(2)，…，Ｖb(n)に変化させている。
【００３６】
このようなパッチ作成条件で図８に示すような１ｏｎ５ｏｆｆのラインパッチ画像ＬＩを感光体２上に順次形成しながら、各パッチ画像を中間転写ベルト７１の外周面に一次転写する（ステップＳ２２３）。なお、この実施形態では、低濃度側のパッチ画像として１ｏｎ５ｏｆｆのラインパッチ画像ＬＩを形成しているが、このラインパッチ画像に限定されるものではなく、種々のハーフトーン画像をパッチ画像として用いることができる。ただし、パッチ画像ＬＩを低濃度側のパッチ画像として用いた場合には、次のような作用効果が得られる。
【００３７】
従来、線画像の画像濃度を調整するために、例えば特開平９−５０１５５号公報に記載の発明では、３ドットラインのペア群を３ドットおきに出力してなるパッチ画像を用いており、このパッチ画像をセンサによって読み取ることでライン幅を検出している。そして、こうして検出されるライン幅に基づき、レーザーパワーを制御することで所望のライン幅が得られるように露光量を調整し、理想のライン線画像を得ている。しかしながら、線画像の基本はレーザービーム１本で描画される１ドットラインであり、従来例の如く複数ドットラインのライン幅を制御しただけでは線画像を十分に調整したとはいえない。これに対し、本実施形態では、図８に示すように、互いに離隔配置された複数本の１ドットラインで構成されるトナー像がラインパッチ画像として形成される。そして、後述するように、このラインパッチ画像の光学濃度を測定し、低濃度側目標濃度となるように調整しているので、１ドットラインからなる線画像の画像濃度を安定化させることができる。
【００３８】
また、ラインパッチ画像が１ｏｎ５ｏｆｆとなっている点でも有利な作用効果が得られる。光ビームＬは一般的にガウス型の光強度分布を有しており、通常光強度の最大値に対して約５０％レベルでのスポット径が設計解像度に対応するように設計スポット径を調整することが多いが、この場合、露光エネルギーとして有効な１／ｅ^２に対応する有効露光スポット径は設計スポット径よりも大きくなることから、隣接する１ドットライン同士のライン間隔が狭い場合には、隣接する有効露光スポット同士が干渉し合い、その干渉により、ライン間の表面電位を変化させてしまうという問題である。これによって、元来１ドットラインとして描画したラインの線幅が太ってしまう。これに対し、本実施形態の如く隣接する１ドットラインの間に５ライン分の間隔を設けることによって、かかる問題を解消することができる。つまり、隣接するラインの影響を受けずに、孤立１ドットライン群を形成することができる。もちろん、６ライン以上離間させるように構成してもよいのであるが、ｏｆｆ本数が増大すると、それに伴って低濃度側目標濃度を低下させる必要があり、パッチセンサＰＳの測定感度の面で問題がある場合がある。したがって、本実施形態の如くラインパッチ画像として１ｏｎ５ｏｆｆを用いることはこれらの点を総合的に考慮すると、最も効果的なパッチ画像といえる。
【００３９】
なお、パッチセンサの測定感度を上げるという観点で、低濃度側目標濃度を上げる場合は、例えば、横１ｏｎ５ｏｆｆ、縦１ｏｎ５ｏｆｆのいわゆる格子パターンが孤立１ドットライン群を形成するという意味で好ましい。この格子パターンにおいても、上記と同様に、縦および横のいずれにおいても６ライン以上離間させてもよいことは言うまでもない。
【００４０】
次のステップＳ２２４では、すべてのパッチ作成色についてパッチ画像を作成したか否かを判断し、「ＮＯ」と判断される間は、パッチ作成色を次の色に設定し（ステップＳ２２５）、ステップＳ２２２，Ｓ２２３を繰り返して他のトナー色、つまりシアン（Ｃ）、マゼンタ（Ｍ）、イエロー（Ｙ）のラインパッチ画像を中間転写ベルト７１の外周面上にさらに形成していく。
【００４１】
一方、ステップＳ２２４で「ＹＥＳ」と判断すると、各ラインパッチ画像の光学濃度をパッチセンサＰＳで測定する（ステップＳ２２６）。
【００４２】
これに続いて、ステップＳ２２７で低濃度側目標濃度（ＯＤ＝０．２）と一致するパッチ作成条件を「低濃度側相関データ」として抽出する。例えば図５（ｂ）および６（ｂ）に示すように、各パッチ作成条件（Ｅ，Ｖb）で作成されたラインパッチ画像のうち、パッチ作成条件（Ｅ1，Ｖb(2)）、（Ｅ2，Ｖb(3)）、…などで作成されたラインパッチ画像が低濃度側目標濃度と一致する場合、これら（Ｅ1，Ｖb(1)）、（Ｅ2，Ｖb(3)）、…が低濃度側相関データとなる。
【００４３】
このようにして通常用およびセーブ用高濃度側相関データ、ならびに低濃度側相関データが求まると、ステップＳ２３で通常用高濃度側相関データと低濃度側相関データの積集合を求める。例えば図５に示すように高濃度側相関データおよび低濃度側相関データがそれぞれ求められた場合、通常用高濃度側相関データおよび低濃度側相関データの積集合は相関データ（Ｅ2，Ｖb(3)）となる。そこで、露光エネルギーＥ2および現像バイアスＶb(3)をそれぞれ通常モードにおける最適露光エネルギーおよび最適現像バイアスとして設定し、メモリ１２７に記憶する。
【００４４】
また、ステップＳ２４でセーブ用高濃度側相関データと低濃度側相関データの積集合を求める。例えば図６に示すように高濃度側相関データおよび低濃度側相関データがそれぞれ求められた場合、セーブ用高濃度側相関データおよび低濃度側相関データの積集合は相関データ（Ｅm，Ｖb(n)）となる。そこで、露光エネルギーＥmおよび現像バイアスＶb(n)をそれぞれトナーセーブモードにおける最適露光エネルギーおよび最適現像バイアスとして設定し、１２７に記憶する。
【００４５】
なお、この実施形態では、通常モードおよびトナーセーブモードのいずれにおいても、積集合に属する相関データが単一の相関データのみとなっている場合を例示して説明したが、複数の相関データが存在する場合には積集合に属する一の相関データを選択し、その相関データを構成する露光エネルギーおよび現像バイアスをそれぞれ最適露光エネルギーおよび最適現像バイアスとして設定すればよい。
【００４６】
上記のようにして通常モードおよびトナーセーブモードにおける濃度調整因子の最適値（最適露光エネルギーおよび最適現像バイアス）が設定記憶された後、実際の印字処理が行われるが、通常モードが選択されているときにはメモリ１２７から最適露光エネルギーＥ2および最適現像バイアスＶb(3)が読み出され、露光エネルギーおよび現像バイアスとしてそれぞれ設定された後、印字が実行される。一方、トナーセーブモードが選択されているときにはメモリ１２７から最適露光エネルギーＥmおよび最適現像バイアスＶb(n)が読み出され、露光エネルギーおよび現像バイアスとしてそれぞれ設定された後、印字が実行される。
【００４７】
以上のように、この実施形態によれば、トナーセーブモードにおいては、基準目標濃度よりも低いトナーセーブ用目標濃度でトナー像を形成しているため、トナーセーブモードにおけるトナー消費量を通常モードのときよりも削減することができる。また、トナーセーブモードにおける画像品質について着目すると、次のような作用効果が得られる。
【００４８】
この実施形態では、原画像のドットイメージの中からドットを間引くことなく、画像濃度を調整することによってトナー消費量の抑制を図っているため、通常モードにおいて形成される画像表現を維持しながら、トナー消費量を抑制しつつ画像形成を行うことができる。
【００４９】
また、トナー消費量の低減を図るためには、全濃度領域に渡って一律に画像濃度を低下させることも考えられるが、このように画像濃度を低下させた場合には文字や細線などの低濃度の画像がきちんと再現されず、例えばかすれて判別することが難しくなるケースがある。これに対し、本実施形態によれば、低濃度側については通常モードと同じ目標濃度（光学濃度ＯＤ＝０．２）で各色のトナー像を形成しているため、特に文字や細線などの低濃度の画像については通常モードと遜色ない判別性を確保することができる。その一方で、高濃度側については目標濃度を「１．２」から「０．８」に低下させることによってトナー消費量を積極的に抑制して効率的なトナーセーブが可能となっている。
【００５０】
さらに、電子写真方式の画像形成装置では、装置の個体差、感光体およびトナーの疲労・経時変化や、装置周辺における温湿度の変化などに起因して、トナー像の画像濃度が異なることがあるが、上記実施形態では、トナーセーブモードを実行するにあたって、そのモードにおける濃度調整因子の最適値をいわゆるパッチセンシング処理によって求めているため、装置の個体差などの影響を受けることなく、低濃度から高濃度までの広い濃度範囲においてトナーセーブ用目標濃度にて画像を安定して形成することができる。
【００５１】
Ｂ．第２実施形態
ところで、上記第１実施形態では、トナーセーブモードにおける濃度調整因子の最適値をパッチ画像の画像濃度に基づき直接求めているが、以下に説明するようにいわゆるパッチセンシング処理によって求められた通常モードにおける濃度調整因子の最適値を補正することによってトナーセーブモードにおける濃度調整因子の最適値を求めてもよい。つまり、この第２実施形態では、パッチ画像に基づき間接的にトナーセーブモードにおける濃度調整因子の最適値を求めている。以下、図９ないし図１２を参照しつつ第２実施形態について説明する。なお、この第２実施形態にかかる画像形成装置の機械的および電気的構成は第１実施形態のそれと同一であるため、同一符号を付して、それら構成に関する説明は省略する。
【００５２】
図９は、第２実施形態における濃度調整因子の最適化処理を示すフローチャートである。この第２実施形態では、まず、パッチセンシング処理によって、高濃度側目標濃度（ＯＤ＝１．２）でトナー像を形成するための露光エネルギーＥと現像バイアスＶbとの相関データを「通常用高濃度側相関データ」として求める（ステップＳ２１）とともに、低濃度側目標濃度（ＯＤ＝０．２）でトナー像を形成するための露光エネルギーＥと現像バイアスＶbとの相関データを低濃度側相関データとして求め（ステップＳ２２）、それに続いて、通常用高濃度側相関データと低濃度側相関データの積集合を求めて通常モードにおける濃度調整因子の最適値（最適露光エネルギーＥ2および最適現像バイアスＶb(3)）を求め、メモリ１２７に記憶する（ステップＳ２３）。ここまでの一連のパッチセンシング処理については、第１実施形態のそれらと同一であるため、ここでは説明を省略する。
【００５３】
次に、上記のようにして求められた通常モードでの最適露光エネルギーＥ2および最適現像バイアスＶb(3)を感光体２の光減衰特性に基づいて補正してトナーセーブモードでの濃度調整因子の最適値（最適露光エネルギーおよび最適現像バイアス）を求める（ステップＳ２５）。具体的には、露光エネルギーを通常モードでの最適露光エネルギーＥ2よりも高い値に補正するとともに、現像バイアスの絶対値を通常モードでの最適現像バイアスＶb(3)の絶対値よりも低い値に補正する。このように補正する理由は以下の通りである。
【００５４】
図１０は、感光体の光減衰特性と、高濃度側相関データとの関係を示すグラフであり、実線は感光体２の光減衰特性を示している。同図の実線からわかるように、帯電ユニット３によって帯電処理された感光体２は表面電位Ｖoを有しており、この感光体２に向けて露光エネルギーＥで光ビームＬを照射してベタ画像に対応する潜像が形成された場合、その潜像部分の表面電位は明部電位Ｖonとなる。
【００５５】
ところで、高濃度側目標濃度のトナー像を得るためには、感光体２の表面に所定のトナー付着量を付着させる必要（目標濃度とトナー付着量との相関は既知）がある。ここで、基準目標濃度の高濃度側目標濃度（光学濃度ＯＤ＝１．２）でトナー像を形成するために所定のトナー付着量が必要となり、そのトナー付着量に対応するコントラスト電位（＝│（現像バイアス）−（感光体２の明部電位）│）を電位Ｖconとすれば、同図の１点鎖線に示すように、明部電位Ｖonと現像バイアスＶbとの差がコントラスト電位Ｖconとなるように、露光エネルギーＥと現像バイアスＶbを設定することによって高濃度側目標濃度でトナー像を形成することができる。つまり、上記ステップＳ２１では、感光体２の光減衰特性に基づいて図１０の１点鎖線で示す通常用高濃度側相関データを求めているのに他ならない。また、ステップＳ２２ではパッチセンシング処理によって低濃度側相関データを求めているが、これも上記高濃度側相関データと同様に感光体２の光減衰特性に基づいて図１１の２点鎖線で示す低濃度側相関データを求めているに他ならない。したがって、通常モードにおける最適値については、図１２に示すように、高濃度側相関データを表す１点鎖線と、低濃度側相関データを表す２点鎖線との交点ＣＰnを求め、この交点ＣＰnに対応する露光エネルギーＥ2および現像バイアスＶb(3)をそれぞれ最適露光エネルギーおよび最適現像バイアスとして設定し、メモリ１２７に記憶することができる（ステップＳ２３）。
【００５６】
これに対して、トナーセーブ用目標濃度の高濃度側目標濃度（光学濃度ＯＤ＝０．８）でトナー像を形成するために必要となるトナー付着量は通常用目標濃度の場合よりも少なく、必然的にそのトナー付着量に対応するコントラスト電位も小さくなる。ここで、そのコントラスト電位を電位Ｖcon′（＜Ｖcon）とすれば、図１０の破線に示すように、明部電位Ｖonと現像バイアスＶbとの差がコントラスト電位Ｖcon′となるように、露光エネルギーＥと現像バイアスＶbを設定することによってトナーセーブ用の高濃度側目標濃度でトナー像を形成することができ、この破線がセーブ用高濃度側相関データを示しているのに他ならない。したがって、トナーセーブモードでの最適値は、図１２に示すように、セーブ用高濃度側相関データ（破線）と低濃度側相関データ（２点鎖線）との交点ＣＰsとなる。つまり、トナーセーブモードでの最適露光エネルギーについては通常モードでの最適露光エネルギーＥ2よりも高い値Ｅmとなり、トナーセーブモードでの現像バイアスの絶対値は通常モードでの最適現像バイアスＶb(3)の絶対値よりも低い値｜Ｖb(n)｜となる。
【００５７】
よって、パッチセンシング処理によりセーブ用高濃度側相関データを求めるまでもなく、トナーセーブ用の高濃度目標濃度が決まっておれば、感光体２の光減衰特性に基づいて上記のように通常モードでの最適値を補正することによってトナーセーブモードでの最適値を求めることができる。
【００５８】
以上のように、この第２実施形態においても、第１実施形態と同様に、トナーセーブモードにおいては、基準目標濃度よりも低いトナーセーブ用目標濃度でトナー像を形成しているため、トナーセーブモードにおけるトナー消費量を通常モードのときよりも削減することができるとともに、トナーセーブモードにおける画像品質についても第１実施形態と同様の作用効果が得られる。
【００５９】
また、この第２実施形態では、セーブ用高濃度側相関データを求めることなく、通常モードでの最適値を単に補正することによってトナーセーブモードでの最適値を求めているため、第１実施形態に比べて演算処理量が少なく、制御ユニット１に与える負荷を低減することができる。なお、トナーセーブモードの最適値を求めるにあたってパッチ画像の画像濃度に基づき直接トナーセーブモードにおける濃度調整因子の最適値を求めているわけではないが、パッチセンシング処理により求めた通常モードでの最適値に基づき求めており、パッチ画像の画像濃度を間接的に利用している。そのため、第１実施形態とほぼ同様に、装置の個体差などの影響を受けることなく、低濃度から高濃度までの広い濃度範囲においてトナーセーブ用目標濃度にて画像を安定して形成することができる。
【００６０】
Ｃ．その他
なお、本発明は上記した実施形態に限定されるものではなく、その趣旨を逸脱しない限りにおいて上述したもの以外に種々の変更を行うことが可能である。例えば、上記実施形態では、各モードにおける高濃度側目標濃度として光学濃度ＯＤ＝１．２，０．８を用いるとともに、各モードにおいて低濃度側目標濃度として光学濃度ＯＤ＝０．２を共通的に用いているが、各目標濃度の設定値はこれに限定されるものではなく、トナーセーブ用目標濃度を基準目標濃度よりも低く設定することを条件に各目標濃度を任意に設定することができる。
【００６１】
また、上記実施形態では、基準目標濃度およびトナーセーブ用目標濃度はともに２種類の高濃度側目標濃度および低濃度側目標濃度を有しているが、基準目標濃度およびトナーセーブ用目標濃度はともに単一種類や３種類以上の目標濃度で構成されていてもよいことは言うまでもない。
【００６２】
また、上記実施形態では、濃度調整因子として光ビームＬの露光エネルギーおよび現像バイアスを用いているが、濃度調整因子の種類や個数などについては任意であり、濃度調整因子としては露光エネルギーおよび現像バイアス以外に帯電バイアスや転写バイアスなどが含まれ、また一の濃度調整因子のみを制御したり、３種類以上の濃度調整因子を組み合わせて制御する画像形成装置にも、本発明を適用することができる。
【００６３】
また、上記実施形態では、４色のトナーを用いたカラー画像を形成することができる画像形成装置であったが、本発明の適用対象はこれに限定されるものではなく、モノクロ画像のみを形成する画像形成装置にも当然に適用することができる。また、上記実施形態にかかる画像形成装置は、ホストコンピュータなどの外部装置よりインターフェース１１２を介して与えられた画像を複写紙、転写紙、用紙およびＯＨＰ用透明シートなどのシートに形成するプリンタであるが、本発明は複写機やファクシミリ装置などの電子写真方式の画像形成装置全般に適用することができる。
【００６４】
また、上記実施形態では、感光体２上のトナー像を中間転写ベルト７１に転写し、このトナー像をパッチ画像として、その光学濃度を検出するとともに、その検出結果に基づき最適露光エネルギーおよび最適現像バイアスを求めているが、中間転写ベルト以外の転写媒体（転写ドラム、転写ベルト、転写シート、中間転写ドラム、中間転写シート、反射型記録シートあるいは透過性記憶シートなど）にトナー像を転写してパッチ画像を形成する画像形成装置にも本発明を適用することができる。
【００６５】
また、上記実施形態では、本発明の「転写媒体」として機能する中間転写ベルト７１上に複数個のパッチ画像を並べて形成し、パッチセンサＰＳによって各パッチ画像の光学濃度を一括して測定しているが、パッチ画像を中間転写ベルト７１に一次転写するたびにパッチ画像の光学濃度を測定したり、パッチ画像をいくつかのブロックに分割し、各ブロックを一括して測定したり、あるいは、パッチセンサＰＳとは異なる濃度読取り用のセンサを本発明の「濃度測定手段」として感光体２の外周面に沿って配置して該センサにより感光体２上に形成されたパッチ画像の光学濃度を測定するようにしてもよい。
【００６６】
さらに、上記実施形態では、高濃度パッチ形成後に低濃度パッチを形成しているが、これらが混合したパッチを形成し、センシングすることも可能である。
【００６７】
【発明の効果】
以上のように、この発明によれば、トナーセーブモードにおいて、通常モードよりも低い画像濃度となるように濃度調整因子としての露光エネルギーおよび現像バイアスを制御してトナー像を形成しているため、通常モードにおいて形成される画像表現を維持しながら、トナーの消費量を抑制することができる。
【００６８】
特に、この発明の一の態様のトナーセーブモードでは、通常モードよりも低濃度の高濃度側目標濃度に対応する高濃度側相関データと、通常モードと同じ低濃度側目標濃度に対応する低濃度側相関データとの積集合に属する一の相関データを最適露光エネルギーおよび最適現像バイアスとして設定する。そのため、特に文字や細線などの低濃度の画像については通常モードと遜色ない判別性を確保しながらも、トナーの消費量を抑制することができる。
【００６９】
また、この発明の他の態様のトナーセーブモードにおいては、通常モードにおける最適露光エネルギーおよび最適現像バイアスに対して、露光エネルギーをより高く、現像バイアスをその絶対値がより低くなるように設定する。このとき設定される露光エネルギーおよび現像バイアスは低濃度側相関データに属するものである。そのため、上記と同様に、特に文字や細線などの低濃度の画像については通常モードと遜色ない判別性を確保しながらも、トナーの消費量を抑制することができる。
【図面の簡単な説明】
【図１】この発明にかかる画像形成装置の第１実施形態を示す図である。
【図２】図１の画像形成装置の電気的構成を示すブロック図である。
【図３】図１の画像形成装置における濃度調整因子の最適化処理を示すフローチャートである。
【図４】第１実施形態における高濃度側相関データの導出手順を示すフローチャートである。
【図５】通常モードでの濃度調整因子の最適値を求める手順を示す図である。
【図６】トナーセーブモードでの濃度調整因子の最適値を求める手順を示す図である。
【図７】第１実施形態における低濃度側相関データの導出手順を示すフローチャートである。
【図８】ライン画像を模式的に示す図である。
【図９】第２実施形態における濃度調整因子の最適化処理を示すフローチャートである。
【図１０】感光体の光減衰特性と、高濃度側相関データとの関係を示すグラフである。
【図１１】感光体の光減衰特性と、低濃度側相関データとの関係を示すグラフである。
【図１２】通常モードでの濃度調整因子の最適値と、トナーセーブモードでの濃度調整因子の最適値との相対関係を示す図である。
【符号の説明】
１…制御ユニット（制御手段）
２…感光体
４Ｋ，４Ｃ，４Ｍ，４Ｙ…現像器（現像手段）
６…露光ユニット（露光手段）
１２…エンジンコントローラ（制御手段）
７１…中間転写ベルト（転写媒体）
１２４…ＣＰＵ（制御手段）
Ｅ…露光エネルギー
Ｌ…光ビーム
ＬＩ…ラインパッチ画像
ＰＳ…パッチセンサ（濃度検出手段）[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus and an image forming method for forming a toner image by exposing a surface of a photoreceptor to a light beam to form an electrostatic latent image and then developing the electrostatic latent image with toner. It is.
[0002]
[Prior art]
Conventionally, in an image forming apparatus such as a printer, a copying machine, or a facsimile using an electrophotographic method, a photosensitive member is charged by a charging unit, and then a light beam is exposed on the surface of the photosensitive member to form an electrostatic latent image. The electrostatic latent image is developed with toner (developer) by a developing unit to be visualized, and then the toner image is transferred to a sheet such as copy paper, transfer paper, paper, and an OHP transparent sheet, and a fixing device The image is formed by fixing the image.
[0003]
In such an image forming apparatus, an apparatus having a toner save mode has been conventionally proposed in order to reduce toner consumption while maintaining image expression. Specifically, there has been conventionally known an apparatus that reduces the amount of toner consumption by thinning out dots from a dot image of an original image at a certain rate when the toner save mode is selected.
[0004]
[Problems to be solved by the invention]
However, since dots are thinned out from the dot image in the conventional toner save mode, data is lost in a low-density image of characters and fine lines, making it impossible to distinguish characters and the like. Also, for a high density image such as a solid image, the image is expressed in a halftone dot shape in the toner save mode, so that gradation is not sufficient and the image becomes unnatural. As described above, there is a problem that the image representation cannot be substantially maintained by simply thinning out dots in the toner save mode.
[0005]
The present invention has been made in view of the above problems, and an image forming apparatus capable of performing image formation while suppressing toner consumption while maintaining image representation formed in the normal mode even when the toner save mode is selected. A first object is to provide an image forming method.
[0006]
A second object of the present invention is to suppress toner consumption while securing distinctiveness comparable to that of the normal mode, particularly for low-density images such as characters and fine lines.
[0007]
[Means for Solving the Problems]
An image forming apparatus according to the present invention includes an exposure unit that exposes a surface of a photoreceptor with a light beam to form an electrostatic latent image, and the electrostatic latent image on the photoreceptor is visualized with toner to form a toner image. Development means for forming, and control means for controlling the image density of the toner image formed on the photoreceptor by controlling the exposure energy and the development bias as density adjusting factors that affect the image density of the toner image. In order to achieve the first and second objects, a toner image formed on the photoconductor or a toner image formed by transferring the toner image onto a transfer medium is used as a patch image. The image forming apparatus further includes density detecting means for detecting the image density, and the control means includes a normal mode for forming a toner image at a reference target density and a toner save mode for suppressing toner consumption. A low density side target density and a high density side target density as the reference target density, and a high density side target density for toner save that is lower than the high density side target density, A high density patch image is formed while changing the density adjustment factor, and a toner image is formed at the high density side target density based on the image density of the patch image detected by the density detection means. High density side correlation data for normal use between exposure energy and development bias, and high density side correlation data for save between exposure energy and development bias for forming a toner image at the high density side target density for toner save And obtaining a low-density patch image while changing and setting the density adjustment factor, and the patch images detected by the density detection means Based on the image density, low density side correlation data between the exposure energy and development bias for forming a toner image at the low density side target density is obtained. In the normal mode, the low density side correlation data and the normal use data are obtained. One correlation data belonging to the product set with the high density side correlation data is set as the optimum exposure energy and optimum development bias, while in the toner save mode, the low density side correlation data and the save high density side correlation data are set. One correlation data belonging to the product set is set as an optimum exposure energy and an optimum development bias.
[0008]
According to another aspect of the image forming apparatus of the present invention, an exposure unit that forms an electrostatic latent image by exposing a light beam to the surface of the photosensitive member, and the electrostatic latent image on the photosensitive member is exposed with toner. The image density of the toner image formed on the photosensitive member is controlled by controlling the exposure energy and the development bias as a density adjusting factor that affects the image density of the toner image, and a developing unit that forms a toner image by imaging. In order to achieve the first and second objects, a toner image formed on the photosensitive member or the toner image is transferred to a transfer medium. The image forming apparatus further includes density detection means for detecting the image density of the toner image as a patch image. The control means includes a normal mode in which the toner image is formed at a reference target density, and a toner mode for suppressing toner consumption. A save mode, having a low density side target density and a high density side target density as the reference target density, and forming a patch image for high density while changing and setting the density adjustment factor. Based on the image density of the image, the normal high density side correlation data between the exposure energy and the developing bias for forming the toner image at the high density side target density is obtained, and the density adjustment factor is changed and the low density is set. A low density side correlation data between the exposure energy and the development bias for forming a toner image at the low density side target density is obtained based on the image density of the patch image. One correlation data belonging to the product set of the low density side correlation data and the normal high density side correlation data is converted into an optimum exposure energy and an optimum development bin. On the other hand, in the toner save mode, among the correlation data belonging to the low density side correlation data, the exposure energy is higher than the optimal exposure energy in the normal mode, and the absolute value of the developing bias is the normal mode. One correlation data that is lower than the absolute value of the optimum development bias in is set as the optimum exposure energy and the optimum development bias.
[0009]
The correlation data can be expressed, for example, by a combination of the exposure energy and the development bias when the patch image density matches the target density.
[0010]
The low density patch image is preferably a line image composed of a plurality of 1 dot lines spaced apart from each other, for example.
[0011]
Further, the image forming method according to the present invention forms an electrostatic latent image by exposing a surface of a photoreceptor with a light beam, and forms the toner image by developing the electrostatic latent image with toner. In order to achieve the first and second objects, the method includes a normal mode for forming a toner image at a reference target density, and a toner save mode for suppressing toner consumption. It has a low-density side target density and a high-density side target density as reference target densities, and has a high-density side target density for toner save that is lower than the high-density side target density, and affects the image density of the toner image. A high-density patch image is formed while changing and setting exposure energy and development bias as a density adjustment factor that gives the toner image, and a toner image at the target density on the high-density side is based on the image density of those patch images. High-density side correlation data for normal exposure energy and development bias for forming, and high-density side for saving exposure energy and development bias for forming a toner image at the high-density target density for toner save A process for obtaining correlation data, and forming a low-density patch image while changing and setting the density adjustment factor, and forming a toner image at the low-density side target density based on the image density of the patch image A step of obtaining low density side correlation data between exposure energy and development bias, and in the normal mode, one correlation data belonging to a product set of the low density side correlation data and the normal high density side correlation data Is set as the optimum exposure energy and optimum development bias, while in the toner save mode, the low density side correlation data and It is characterized by setting one of the correlation data belonging to the intersection of the save for the high density side correlation data as the optimum exposure energy and an optimal developing bias.
[0012]
According to another aspect of the image forming method of the present invention, an electrostatic latent image is formed by exposing a light beam to the surface of a photoconductor, and the electrostatic latent image is visualized with toner to form a toner image. In order to achieve the first and second objects, the image forming method includes a normal mode for forming a toner image at a reference target density, and a toner save mode for suppressing toner consumption. The reference target density has a low density side target density and a high density side target density, and is used for high density while changing and setting exposure energy and development bias as density adjustment factors that affect the image density of the toner image. Normal high density side correlation data of exposure energy and development bias for forming a patch image and forming a toner image at the high density side target density based on the image density of those patch images A step of obtaining the exposure energy and development for forming a low density patch image while changing and setting the density adjustment factor, and forming a toner image at the low density side target density based on the image density of the patch image A step of obtaining low density side correlation data with a bias, and in the normal mode, one correlation data belonging to a product set of the low density side correlation data and the normal high density side correlation data is determined as an optimum exposure energy. On the other hand, in the toner save mode, the exposure energy is higher than the optimum exposure energy in the normal mode among the correlation data belonging to the low density side correlation data, and the absolute value of the development bias is set in the toner save mode. One correlation data that is lower than the absolute value of the optimum developing bias in the normal mode is obtained as the optimum exposure energy. It is characterized by setting as over and optimum developing bias.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
A. First embodiment
FIG. 1 is a diagram showing a first embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG. This image forming apparatus forms a full color image by superposing four color toners of black (K), cyan (C), magenta (M), and yellow (Y), or uses only black (K) toner. This is an apparatus for forming a monochrome image. In this image forming apparatus, when an image signal is given to the main controller 11 of the control unit 1 from an external device such as a host computer, the engine controller 12 controls each part of the engine unit EG according to a command from the main controller 11. Thus, an image corresponding to the image signal is formed on the sheet S.
[0016]
In this engine unit EG, the photosensitive member 2 is rotatably provided in the arrow direction D1 in the figure. Further, a charging unit 3 as a charging unit, a rotary developing unit 4 as a developing unit, and a cleaning unit 5 are arranged around the photoreceptor 2 along the rotation direction D1. The charging unit 3 is applied with a charging bias from the charging bias generator 121 and uniformly charges the outer peripheral surface of the photoreceptor 2.
[0017]
Then, the light beam L is irradiated from the exposure unit 6 toward the outer peripheral surface of the photosensitive member 2 charged by the charging unit 3. As shown in FIG. 2, the exposure unit 6 is electrically connected to an image signal switching unit 122, and the exposure power control unit 123 performs exposure in accordance with an image signal given through the image signal switching unit 122. The unit 6 is controlled to scan and expose the light beam L onto the photoconductor 2 to form an electrostatic latent image corresponding to the image signal on the photoconductor 2. For example, when the image signal switching unit 122 is electrically connected to the patch creation module 125 based on a command from the CPU 124 of the engine controller 12, the patch image signal output from the patch creation module 125 is sent to the exposure power control unit 123. Given, a patch latent image is formed. On the other hand, when the image signal switching unit 122 is electrically connected to the CPU 111 of the main controller 11, the light beam L is applied to the photoconductor 2 in accordance with an image signal given through an interface 112 from an external device such as a host computer. The electrostatic latent image corresponding to the image signal is formed on the photosensitive member 2 by scanning exposure.
[0018]
The electrostatic latent image thus formed is developed with toner by the developing unit 4. That is, in this embodiment, as the developing unit 4, a black developing device 4K, a cyan developing device 4C, a magenta developing device 4M, and a yellow developing device 4Y are rotatably provided around the axis. . The developing devices 4K, 4C, 4M, and 4Y are rotationally positioned and selectively positioned at a contact position or a separated position with respect to the photosensitive member 2, and a developing bias of a DC component is developed by the developing bias generator 126. Is applied to the surface of the photoreceptor 2 with the selected color toner. As a result, the electrostatic latent image on the photoreceptor 2 is visualized with the selected toner color.
[0019]
The toner image developed by the developing unit 4 as described above is primarily transferred onto the intermediate transfer belt 71 of the transfer unit 7 in the primary transfer region TR1. Further, in the vicinity of the primary transfer region TR1, a patch sensor PS is disposed as a “density measuring unit” of the present invention so as to face the surface of the intermediate transfer belt 71. The optical density of the patch image formed on the outer peripheral surface is measured. Further, a cleaning unit 5 is disposed at a position advanced in the circumferential direction (rotation direction D1 in FIG. 1) from the primary transfer region TR1, and the toner remaining on the outer peripheral surface of the photoreceptor 2 after the primary transfer. Scrap off. If necessary, the charge of the photoreceptor 2 is reset by a charge removal unit (not shown).
[0020]
The transfer unit 7 includes an intermediate transfer belt 71 spanned by a plurality of rollers, and a drive unit (not shown) that rotationally drives the intermediate transfer belt 71. When a color image is transferred to the sheet S, a color image is formed by superimposing the toner images of the respective colors formed on the photoreceptor 2 on the intermediate transfer belt 71, and a predetermined secondary transfer region TR2. 2, the color image is secondarily transferred onto the sheet S taken out from the cassette 8. Further, the sheet S on which the color image is formed in this way is conveyed via the fixing unit 9 to a discharge tray portion provided on the upper surface portion of the apparatus main body.
[0021]
Further, after the secondary transfer, the toner remaining on the outer peripheral surface of the intermediate transfer belt 71 after the secondary transfer is removed from the intermediate transfer belt 71 by a cleaning unit (not shown).
[0022]
In FIG. 2, reference numeral 113 denotes an image memory provided in the main controller 11 for storing an image given from an external device such as a host computer via the interface 112, and reference numeral 127 denotes an arithmetic program executed by the CPU 124. A memory (storage means) for storing calculation results in the CPU 124, control data for controlling the engine unit EG, and the like.
[0023]
Next, the operation of the image forming apparatus configured as described above will be described. In this image forming apparatus, optimization processing for density adjustment factors, which will be described in detail later, is performed at an appropriate timing, for example, when the apparatus power is turned on and when the cumulative count value of the number of printed sheets reaches a predetermined value. The optimum value of the density adjustment factor in the normal mode and the toner save mode is set. Here, the “density adjustment factor” means a factor that affects the image density of the toner image formed on the photoconductor 2, and this first embodiment (and a second embodiment described later). In this case, the exposure energy of the light beam L and the development bias are adopted as density adjustment factors, and these are optimized to adjust the image density of each color toner image.
[0024]
When executing the printing operation, it is determined whether the normal printing is set to “normal mode” or “toner save mode” for suppressing toner consumption, and the former ( When the (normal mode) is selected, the toner image is formed with the reference target density, while when the latter (toner save mode) is selected, the toner image is formed with the toner save target density.
[0025]
In this embodiment, in order to form an image with excellent gradation by adjusting the image density of the toner image from a low density to a high density, each of the reference target density and the toner save target density is set. The low density side target density and the high density side target density are configured. Specifically, as shown in the following table, for the low density side target density, both the reference target density and the toner save target density are set to “optical density OD = 0.2”, and for the high density side target density, While the reference target density is set to “optical density OD = 1.2”, the toner save target density is set to “optical density OD = 0.8”. Here, the difference between the two toner adhesion amounts in the high density target is set to be approximately doubled.
[0026]
[Table 1]

[0027]
FIG. 3 is a flowchart showing the concentration adjustment factor optimization processing in the first embodiment. In the first embodiment, first, correlation data between the exposure energy E and the development bias Vb for forming a toner image at a high density side target density (OD = 1.2) by patch sensing processing is expressed as “normal high "Density-side correlation data" and correlation data between exposure energy E and development bias Vb for forming a toner image at a high-density target density (OD = 0.8) are referred to as "saving high-density side correlation data". Obtained (step S21). More specifically, the control unit 1 controls each part of the apparatus according to the procedure shown in the flowchart shown in FIG. 4 to obtain high density side correlation data.
[0028]
FIG. 4 is a flowchart showing a procedure for deriving high density side correlation data in the first embodiment. First, the color for creating a patch image is set to the first color, for example, black (step S211). Then, the charging bias is set to an initial value stored in advance in the memory 127, while a plurality of patch creation conditions are set (step S212). Here, the exposure energy E is changed to E1, E2,..., Em, and the developing bias Vb is changed to Vb (1), Vb (2),.
[0029]
Each patch image is primarily transferred onto the outer peripheral surface of the intermediate transfer belt 71 while sequentially forming solid patch images (high density patch images) on the photoreceptor 2 under such patch creation conditions (step S213). In this embodiment, a solid patch image is formed. However, the solid patch image is not limited to a solid patch image, and the area ratio of dots with respect to a high density image close to the solid patch image, for example, the entire patch image is about 80% or more. An image may be formed.
[0030]
In the next step S214, it is determined whether or not a patch image has been generated for all patch generation colors. While it is determined “NO”, the patch generation color is set to the next color (step S215). By repeating S212 and S213, solid toner images of other toner colors, that is, cyan (C), magenta (M), and yellow (Y) are further formed on the outer peripheral surface of the intermediate transfer belt 71.
[0031]
On the other hand, if “YES” is determined in the step S214, the optical density of each solid patch image is measured by the patch sensor PS (step S216).
[0032]
Subsequently, in step S217, a patch creation condition that matches the high density side target density (OD = 1.2) of the reference target density is extracted as “normal high density side correlation data”. For example, as shown in FIG. 5A, among the solid patch images created under each patch creation condition (E, Vb), the patch creation conditions (E1, Vb (2)), (E2, Vb (3)), When the solid patch image created in the above-mentioned manner coincides with the high density side target density, these (E1, Vb (2)), (E2, Vb (3)),... Become normal high density side correlation data.
[0033]
In step S218, a patch creation condition that matches the high density side target density (OD = 0.8) of the toner save target density is extracted as “save high density side correlation data”. For example, as shown in FIG. 6A, among the solid patch images created under each patch creation condition (E, Vb), patch creation conditions (E1, Vb (4)), (Em, Vb (n)), When the solid patch image created in the above-mentioned manner matches the high density side target density, these (E1, Vb (4)), (Em, Vb (n)),... Become the high density side correlation data for saving.
[0034]
After obtaining the normal density and save high density side correlation data in this way (step S21), the exposure energy E for forming the toner image at the low density side target density (OD = 0.2) by patch sensing and Correlation data with the developing bias Vb is obtained as low density side correlation data (step S22). More specifically, the control unit 1 controls each part of the apparatus according to the procedure shown in the flowchart shown in FIG. 7, and obtains low concentration side correlation data.
[0035]
FIG. 7 is a flowchart showing a procedure for deriving low-concentration side correlation data in the first embodiment. First, the color for creating a patch image is set to the first color, for example, black (K) (step S221). Then, while setting the charging bias to an initial value, a plurality of patch creation conditions are set (step S222). Here, the exposure energy E is changed to E1, E2,..., Em and the developing bias Vb is changed to Vb (1), Vb (2),. n).
[0036]
While sequentially forming 1 on 5 off line patch images LI as shown in FIG. 8 on the photoreceptor 2 under such patch creation conditions, each patch image is primarily transferred to the outer peripheral surface of the intermediate transfer belt 71 (step S223). In this embodiment, the 1 on 5 off line patch image LI is formed as the low density side patch image. However, the present invention is not limited to this line patch image, and various halftone images can be used as the patch image. Can do. However, when the patch image LI is used as a low-density patch image, the following operational effects can be obtained.
[0037]
Conventionally, in order to adjust the image density of a line image, for example, in the invention described in Japanese Patent Laid-Open No. 9-50155, a patch image obtained by outputting a pair group of 3 dot lines every 3 dots is used. The line width is detected by reading the patch image with a sensor. Based on the line width thus detected, the laser light is controlled to adjust the exposure amount so as to obtain a desired line width, thereby obtaining an ideal line line image. However, the basic of a line image is one dot line drawn by one laser beam, and it cannot be said that the line image is sufficiently adjusted just by controlling the line width of a plurality of dot lines as in the conventional example. On the other hand, in this embodiment, as shown in FIG. 8, a toner image composed of a plurality of one-dot lines spaced apart from each other is formed as a line patch image. As will be described later, since the optical density of the line patch image is measured and adjusted so as to be the low density side target density, the image density of the line image composed of one dot line can be stabilized. .
[0038]
An advantageous effect is also obtained in that the line patch image is 1 on 5 off. The light beam L generally has a Gaussian light intensity distribution, and the design spot diameter is adjusted so that the spot diameter at the level of about 50% of the maximum value of the normal light intensity corresponds to the design resolution. In many cases, 1 / e is effective as exposure energy. ² Since the effective exposure spot diameter corresponding to is larger than the designed spot diameter, if the line interval between adjacent one dot lines is narrow, the adjacent effective exposure spots interfere with each other, and the interference causes the line-to-line It is a problem that the surface potential of the substrate is changed. As a result, the line width originally drawn as a one-dot line becomes thick. On the other hand, this problem can be solved by providing an interval of five lines between adjacent one dot lines as in this embodiment. That is, an isolated 1-dot line group can be formed without being affected by adjacent lines. Of course, it may be configured to be separated by 6 lines or more. However, as the number of off lines increases, it is necessary to decrease the low density side target density accordingly, which causes a problem in the measurement sensitivity of the patch sensor PS. There may be. Therefore, using 1 on 5 off as the line patch image as in the present embodiment can be said to be the most effective patch image considering these points comprehensively.
[0039]
From the viewpoint of increasing the measurement sensitivity of the patch sensor, when increasing the target density on the low density side, for example, a so-called lattice pattern of horizontal 1 on 5 off and vertical 1 on 5 off is preferable in the sense that an isolated 1 dot line group is formed. Needless to say, in this lattice pattern, as in the case described above, the vertical and horizontal lines may be separated by 6 lines or more.
[0040]
In the next step S224, it is determined whether or not a patch image has been generated for all the patch generation colors. While it is determined “NO”, the patch generation color is set to the next color (step S225). By repeating S222 and S223, line patch images of other toner colors, that is, cyan (C), magenta (M), and yellow (Y) are further formed on the outer peripheral surface of the intermediate transfer belt 71.
[0041]
On the other hand, if “YES” is determined in the step S224, the optical density of each line patch image is measured by the patch sensor PS (step S226).
[0042]
Subsequently, in step S227, a patch creation condition that matches the low density side target density (OD = 0.2) is extracted as “low density side correlation data”. For example, as shown in FIGS. 5B and 6B, among the line patch images created under each patch creation condition (E, Vb), the patch creation conditions (E1, Vb (2)), (E2, If the line patch image created by Vb (3)), etc. matches the target density on the low density side, these (E1, Vb (1)), (E2, Vb (3)), ... are on the low density side. Correlation data.
[0043]
When the normal density and save high density side correlation data and the low density side correlation data are obtained in this way, a product set of the normal high density side correlation data and the low density side correlation data is obtained in step S23. For example, when the high-concentration side correlation data and the low-concentration side correlation data are respectively obtained as shown in FIG. 5, the product set of the normal high-concentration side correlation data and the low-concentration side correlation data is the correlation data (E2, Vb (3 )). Therefore, the exposure energy E2 and the development bias Vb (3) are set as the optimum exposure energy and the optimum development bias in the normal mode, respectively, and stored in the memory 127.
[0044]
In step S24, a product set of the high density side correlation data for saving and the low density side correlation data is obtained. For example, when the high-concentration side correlation data and the low-concentration side correlation data are respectively obtained as shown in FIG. 6, the product set of the high-concentration-side correlation data for saving and the low-concentration-side correlation data is the correlation data (Em, Vb (n )). Therefore, the exposure energy Em and the development bias Vb (n) are set as the optimum exposure energy and the optimum development bias in the toner save mode, respectively, and stored in 127.
[0045]
In this embodiment, the case where the correlation data belonging to the product set is only a single correlation data has been described in both the normal mode and the toner save mode. However, there are a plurality of correlation data. In this case, one correlation data belonging to the product set is selected, and the exposure energy and the development bias constituting the correlation data may be set as the optimum exposure energy and the optimum development bias, respectively.
[0046]
After the optimum values (optimum exposure energy and optimum development bias) of the density adjustment factor in the normal mode and the toner save mode are set and stored as described above, actual printing processing is performed, but the normal mode is selected. Sometimes the optimum exposure energy E2 and the optimum development bias Vb (3) are read from the memory 127, set as the exposure energy and the development bias, respectively, and then printed. On the other hand, when the toner save mode is selected, the optimum exposure energy Em and the optimum development bias Vb (n) are read from the memory 127, set as the exposure energy and the development bias, respectively, and then printed.
[0047]
As described above, according to this embodiment, in the toner save mode, the toner image is formed with the target density for toner save that is lower than the reference target density. It can be reduced than when. When attention is paid to the image quality in the toner save mode, the following effects can be obtained.
[0048]
In this embodiment, since the toner consumption is suppressed by adjusting the image density without thinning out dots from the dot image of the original image, while maintaining the image expression formed in the normal mode, Image formation can be performed while suppressing toner consumption.
[0049]
In order to reduce the toner consumption, it is conceivable to reduce the image density uniformly over the entire density region. However, when the image density is reduced in this way, characters, fine lines, and the like are reduced. There is a case where the density image is not reproduced properly, and it becomes difficult to discriminate, for example. In contrast, according to the present embodiment, on the low density side, the toner images of the respective colors are formed at the same target density (optical density OD = 0.2) as in the normal mode. For a density image, it is possible to ensure discrimination that is comparable to the normal mode. On the other hand, on the high density side, by reducing the target density from “1.2” to “0.8”, the toner consumption is positively suppressed and efficient toner saving is possible.
[0050]
Furthermore, in an electrophotographic image forming apparatus, the image density of a toner image may vary due to individual differences of the apparatus, fatigue / aging of the photoconductor and toner, changes in temperature and humidity around the apparatus, and the like. However, in the above embodiment, when the toner save mode is executed, the optimum value of the density adjustment factor in the mode is obtained by so-called patch sensing processing. An image can be stably formed at a target density for toner save in a wide density range up to a high density.
[0051]
B. Second embodiment
By the way, in the first embodiment, the optimum value of the density adjustment factor in the toner save mode is directly obtained based on the image density of the patch image. However, in the normal mode obtained by so-called patch sensing processing as described below. The optimum value of the density adjustment factor in the toner save mode may be obtained by correcting the optimum value of the density adjustment factor. That is, in the second embodiment, the optimum value of the density adjustment factor in the toner save mode is obtained indirectly based on the patch image. Hereinafter, the second embodiment will be described with reference to FIGS. 9 to 12. Since the mechanical and electrical configuration of the image forming apparatus according to the second embodiment is the same as that of the first embodiment, the same reference numerals are given, and descriptions of those configurations are omitted.
[0052]
FIG. 9 is a flowchart showing the optimization process of the concentration adjustment factor in the second embodiment. In the second embodiment, first, correlation data between the exposure energy E and the development bias Vb for forming a toner image at a high density side target density (OD = 1.2) is obtained by “sensing high for normal use” by patch sensing processing. As the “density-side correlation data” (step S21), the correlation data between the exposure energy E and the developing bias Vb for forming the toner image at the low-density target density (OD = 0.2) is obtained as the low-density side correlation data. (Step S22), and subsequently, a product set of the normal high density side correlation data and the low density side correlation data is obtained to obtain the optimum values of the density adjustment factors in the normal mode (optimal exposure energy E2 and optimum development bias Vb ( 3)) is obtained and stored in the memory 127 (step S23). Since a series of patch sensing processes so far are the same as those in the first embodiment, description thereof is omitted here.
[0053]
Next, the optimum exposure energy E2 and the optimum development bias Vb (3) in the normal mode obtained as described above are corrected on the basis of the light attenuation characteristics of the photoreceptor 2, and the density adjustment factor in the toner save mode is corrected. Optimum values (optimum exposure energy and optimum development bias) are obtained (step S25). Specifically, the exposure energy is corrected to a value higher than the optimum exposure energy E2 in the normal mode, and the absolute value of the development bias is set to a value lower than the absolute value of the optimum development bias Vb (3) in the normal mode. to correct. The reason for such correction is as follows.
[0054]
FIG. 10 is a graph showing the relationship between the light attenuation characteristic of the photoconductor and the high density side correlation data, and the solid line shows the light attenuation characteristic of the photoconductor 2. As can be seen from the solid line in the figure, the photosensitive member 2 charged by the charging unit 3 has a surface potential Vo, and a solid image is obtained by irradiating the photosensitive member 2 with the light beam L with the exposure energy E. When the latent image corresponding to is formed, the surface potential of the latent image portion becomes the bright portion potential Von.
[0055]
Incidentally, in order to obtain a toner image having a high density side target density, it is necessary to attach a predetermined toner adhesion amount to the surface of the photoreceptor 2 (the correlation between the target density and the toner adhesion amount is known). Here, a predetermined toner adhesion amount is required to form a toner image at a high density side target density (optical density OD = 1.2) of the reference target density, and a contrast potential (= ||) corresponding to the toner adhesion quantity. If (development bias) − (bright part potential of photoconductor 2) |) is a potential Vcon, the difference between the bright part potential Von and the development bias Vb is the contrast potential Vcon, as shown by the one-dot chain line in FIG. As described above, by setting the exposure energy E and the development bias Vb, a toner image can be formed with a high density side target density. In other words, in step S21, the normal high-density side correlation data indicated by the one-dot chain line in FIG. 10 is obtained based on the light attenuation characteristics of the photosensitive member 2. In step S22, the low density side correlation data is obtained by the patch sensing process. This is also the low density indicated by the two-dot chain line in FIG. 11 based on the light attenuation characteristics of the photoconductor 2 in the same manner as the high density side correlation data. It is nothing but finding concentration-side correlation data. Therefore, for the optimum value in the normal mode, as shown in FIG. 12, an intersection point CPn between the one-dot chain line representing the high-concentration side correlation data and the two-dot chain line representing the low-concentration side correlation data is obtained. The corresponding exposure energy E2 and development bias Vb (3) can be set as the optimum exposure energy and optimum development bias, respectively, and stored in the memory 127 (step S23).
[0056]
On the other hand, the toner adhesion amount required to form a toner image at the high density side target density (optical density OD = 0.8) of the toner save target density is smaller than in the case of the normal target density. Inevitably, the contrast potential corresponding to the toner adhesion amount also decreases. Here, if the contrast potential is the potential Vcon ′ (<Vcon), as shown by the broken line in FIG. 10, the exposure energy is set so that the difference between the bright portion potential Von and the developing bias Vb becomes the contrast potential Vcon ′. By setting E and the developing bias Vb, a toner image can be formed at the high density side target density for toner saving, and this broken line represents the high density side correlation data for saving. Therefore, as shown in FIG. 12, the optimum value in the toner save mode is an intersection CPs between the high density side correlation data for saving (broken line) and the low density side correlation data (two-dot chain line). That is, the optimum exposure energy in the toner save mode is a value Em higher than the optimum exposure energy E2 in the normal mode, and the absolute value of the development bias in the toner save mode is the optimum development bias Vb (3) in the normal mode. The value | Vb (n) | is lower than the absolute value.
[0057]
Therefore, it is not necessary to obtain the high-density side correlation data for saving by the patch sensing process, and if the high-density target density for toner saving is determined, the normal mode is used as described above based on the light attenuation characteristics of the photoreceptor 2. The optimum value in the toner save mode can be obtained by correcting the optimum value.
[0058]
As described above, also in the second embodiment, similarly to the first embodiment, in the toner save mode, the toner image is formed with the target density for toner save lower than the reference target density. The toner consumption amount in the mode can be reduced as compared with the normal mode, and the same effect as the first embodiment can be obtained with respect to the image quality in the toner save mode.
[0059]
In the second embodiment, since the optimum value in the toner save mode is obtained by simply correcting the optimum value in the normal mode without obtaining the high-density-side correlation data for saving, the first embodiment is obtained. Compared to the above, the amount of calculation processing is small, and the load applied to the control unit 1 can be reduced. Note that the optimum value of the density adjustment factor in the toner save mode is not directly obtained based on the image density of the patch image when obtaining the optimum value of the toner save mode, but the optimum value in the normal mode obtained by the patch sensing process. The image density of the patch image is indirectly used. Therefore, as in the first embodiment, an image can be stably formed at a target density for toner save in a wide density range from a low density to a high density without being affected by individual differences among apparatuses. it can.
[0060]
C. Other
The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, in the above embodiment, the optical density OD = 1.2, 0.8 is used as the high density side target density in each mode, and the optical density OD = 0.2 is commonly used as the low density side target density in each mode. However, the setting value of each target density is not limited to this, and each target density can be arbitrarily set on condition that the toner saving target density is set lower than the reference target density. it can.
[0061]
In the above embodiment, both the reference target density and the toner save target density have two types of the high density side target density and the low density side target density. Needless to say, it may be composed of a single type or three or more types of target densities.
[0062]
In the above embodiment, the exposure energy and the development bias of the light beam L are used as the density adjustment factors, but the type and number of density adjustment factors are arbitrary, and the exposure energy and the development bias are used as the density adjustment factors. The present invention can also be applied to an image forming apparatus that includes a charging bias, a transfer bias, and the like, controls only one density adjustment factor, or controls by combining three or more density adjustment factors. .
[0063]
In the above embodiment, the image forming apparatus is capable of forming a color image using four colors of toner. However, the application target of the present invention is not limited to this, and only a monochrome image is formed. Of course, the present invention can also be applied to an image forming apparatus. The image forming apparatus according to the above embodiment is a printer that forms an image given from an external device such as a host computer via an interface 112 on a sheet such as copy paper, transfer paper, paper, and an OHP transparent sheet. However, the present invention can be applied to all electrophotographic image forming apparatuses such as copying machines and facsimile machines.
[0064]
In the above embodiment, the toner image on the photosensitive member 2 is transferred to the intermediate transfer belt 71, and the optical density is detected using the toner image as a patch image. Based on the detection result, the optimum exposure energy and the optimum development are detected. We are seeking bias, but transfer the toner image to a transfer medium other than the intermediate transfer belt (transfer drum, transfer belt, transfer sheet, intermediate transfer drum, intermediate transfer sheet, reflective recording sheet, or transmissive storage sheet). The present invention can also be applied to an image forming apparatus that forms a patch image.
[0065]
In the above embodiment, a plurality of patch images are formed side by side on the intermediate transfer belt 71 functioning as the “transfer medium” of the present invention, and the optical density of each patch image is collectively measured by the patch sensor PS. However, every time the patch image is primarily transferred to the intermediate transfer belt 71, the optical density of the patch image is measured, the patch image is divided into several blocks, and each block is measured at once, or the patch A density reading sensor different from the sensor PS is arranged along the outer peripheral surface of the photosensitive member 2 as the “density measuring means” of the present invention, and the optical density of the patch image formed on the photosensitive member 2 is measured by the sensor. You may make it do.
[0066]
Furthermore, in the above embodiment, the low density patch is formed after the high density patch is formed, but it is also possible to form a patch in which these are mixed and sense.
[0067]
【The invention's effect】
As described above, according to the present invention, in the toner save mode, the toner image is formed by controlling the exposure energy and the development bias as the density adjustment factor so that the image density is lower than that in the normal mode. It is possible to suppress toner consumption while maintaining the image expression formed in the normal mode.
[0068]
In particular, in the toner save mode according to one aspect of the present invention, the high density side correlation data corresponding to the high density side target density lower than that in the normal mode, and the low density corresponding to the same low density side target density as in the normal mode. One correlation data belonging to the product set with the side correlation data is set as the optimum exposure energy and the optimum development bias. Therefore, particularly for low-density images such as characters and fine lines, it is possible to suppress toner consumption while ensuring discrimination that is comparable to the normal mode.
[0069]
In the toner save mode according to another aspect of the present invention, the exposure energy is set higher than the optimum exposure energy and optimum development bias in the normal mode, and the development bias is set to have a lower absolute value. The exposure energy and development bias set at this time belong to the low density side correlation data. Therefore, in the same manner as described above, it is possible to suppress the toner consumption while securing distinctiveness that is comparable to the normal mode particularly for low-density images such as characters and fine lines.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a first embodiment of an image forming apparatus according to the present invention.
2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG.
FIG. 3 is a flowchart showing density adjustment factor optimization processing in the image forming apparatus of FIG. 1;
FIG. 4 is a flowchart showing a procedure for deriving high density side correlation data in the first embodiment.
FIG. 5 is a diagram illustrating a procedure for obtaining an optimum value of a concentration adjustment factor in a normal mode.
FIG. 6 is a diagram illustrating a procedure for obtaining an optimum value of a density adjustment factor in a toner save mode.
FIG. 7 is a flowchart showing a procedure for deriving low-concentration side correlation data in the first embodiment.
FIG. 8 is a diagram schematically showing a line image.
FIG. 9 is a flowchart showing concentration adjustment factor optimization processing in the second embodiment;
FIG. 10 is a graph showing a relationship between a light attenuation characteristic of a photoconductor and high density side correlation data.
FIG. 11 is a graph showing the relationship between the light attenuation characteristics of a photoconductor and low density side correlation data.
FIG. 12 is a diagram illustrating a relative relationship between an optimum value of a density adjustment factor in a normal mode and an optimum value of a density adjustment factor in a toner save mode.
[Explanation of symbols]
1 ... Control unit (control means)
2 ... Photoconductor
4K, 4C, 4M, 4Y ... developer (developing means)
6 ... exposure unit (exposure means)
12 ... Engine controller (control means)
71. Intermediate transfer belt (transfer medium)
124 ... CPU (control means)
E ... Exposure energy
L ... Light beam
LI ... Line patch image
PS ... Patch sensor (Density detection means)

Claims (6)

An exposure unit that exposes the surface of the photoconductor to form an electrostatic latent image; a developing unit that visualizes the electrostatic latent image on the photoconductor with toner to form a toner image; An image forming apparatus comprising: a control unit that controls an image density of a toner image formed on the photosensitive member by controlling an exposure energy and a developing bias as a density adjusting factor that affects the image density;
A toner image formed on the photoconductor or a toner image formed by transferring the toner image onto a transfer medium as a patch image, further comprising a density detection means for detecting the image density;
The control means has a normal mode for forming a toner image at a reference target density, and a toner save mode for suppressing toner consumption,
The reference target density includes a low density side target density and a high density side target density, and has a toner density high density side target density lower than the high density side target density,
A high density patch image is formed while changing the density adjustment factor, and a toner image is formed at the high density side target density based on the image density of the patch image detected by the density detection means. High density side correlation data for normal use between exposure energy and development bias, and high density side correlation data for save between exposure energy and development bias for forming a toner image at the high density side target density for toner save Asking
A low density patch image is formed while changing the density adjustment factor, and a toner image is formed at the low density side target density based on the image density of the patch image detected by the density detection means. Obtain low density side correlation data between exposure energy and development bias,
In the normal mode, one correlation data belonging to the product set of the low density side correlation data and the normal high density side correlation data is set as the optimum exposure energy and the optimum development bias.
In the toner save mode, one correlation data belonging to a product set of the low density side correlation data and the high density side correlation data for saving is set as an optimum exposure energy and an optimum development bias. apparatus. An exposure unit that exposes the surface of the photoconductor to form an electrostatic latent image; a developing unit that visualizes the electrostatic latent image on the photoconductor with toner to form a toner image; An image forming apparatus comprising: a control unit that controls an image density of a toner image formed on the photosensitive member by controlling an exposure energy and a developing bias as a density adjusting factor that affects the image density;
A toner image formed on the photoconductor or a toner image formed by transferring the toner image onto a transfer medium as a patch image, further comprising a density detection means for detecting the image density;
The control means has a normal mode for forming a toner image at a reference target density, and a toner save mode for suppressing toner consumption,
The reference target density has a low density side target density and a high density side target density,
A high density patch image is formed while changing the density adjustment factor, and a toner image is formed at the high density side target density based on the image density of the patch image detected by the density detection means. While obtaining the normal high density side correlation data of exposure energy and development bias,
A low density patch image is formed while changing the density adjustment factor, and a toner image is formed at the low density side target density based on the image density of the patch image detected by the density detection means. Obtain low density side correlation data between exposure energy and development bias,
In the normal mode, one correlation data belonging to the product set of the low density side correlation data and the normal high density side correlation data is set as the optimum exposure energy and the optimum development bias.
In the toner save mode, among the correlation data belonging to the low density side correlation data, the exposure energy is higher than the optimum exposure energy in the normal mode, and the absolute value of the development bias is the absolute value of the optimum development bias in the normal mode. An image forming apparatus characterized by setting one correlation data lower than a value as an optimum exposure energy and an optimum development bias.   The image forming apparatus according to claim 1, wherein the correlation data is represented by a combination of the exposure power and the developing bias when the patch image density matches the target density.   The image forming apparatus according to claim 1, wherein the low-density patch image is a line image including a plurality of one-dot lines that are spaced apart from each other. In an image forming method in which a light beam is exposed on the surface of a photoreceptor to form an electrostatic latent image, and the electrostatic latent image is visualized with toner to form a toner image.
A normal mode for forming a toner image at a reference target density, and a toner save mode for suppressing toner consumption;
The reference target density includes a low density side target density and a high density side target density, and has a toner density high density side target density lower than the high density side target density,
A high-density patch image is formed while changing and setting exposure energy and developing bias as a density adjustment factor that affects the image density of the toner image, and the toner at the target density on the high density side is formed based on the image density of the patch image. Normal high-density side correlation data between exposure energy for forming an image and development bias, and high saving energy for exposure energy and development bias for forming a toner image at the high-density side target density for toner save A step of obtaining concentration-side correlation data,
A low density patch image is formed while changing the density adjustment factor, and a low exposure energy and a developing bias for forming a toner image at the low density side target density based on the image density of the patch image. A step of obtaining concentration side correlation data,
In the normal mode, one correlation data belonging to the product set of the low density side correlation data and the normal high density side correlation data is set as the optimum exposure energy and the optimum development bias, while in the toner save mode, An image forming method, wherein one correlation data belonging to a product set of the low density side correlation data and the high density side correlation data for saving is set as an optimum exposure energy and an optimum development bias. In an image forming method in which a light beam is exposed on the surface of a photoreceptor to form an electrostatic latent image, and the electrostatic latent image is visualized with toner to form a toner image.
A normal mode for forming a toner image at a reference target density, and a toner save mode for suppressing toner consumption;
The reference target density has a low density side target density and a high density side target density,
A high-density patch image is formed while changing and setting exposure energy and developing bias as a density adjustment factor that affects the image density of the toner image, and the toner at the target density on the high density side is formed based on the image density of the patch image. Obtaining normal high density side correlation data between exposure energy and development bias for forming an image;
A low density patch image is formed while changing the density adjustment factor, and a low exposure energy and a developing bias for forming a toner image at the low density side target density based on the image density of the patch image. A step of obtaining concentration side correlation data,
In the normal mode, one correlation data belonging to the product set of the low density side correlation data and the normal high density side correlation data is set as the optimum exposure energy and the optimum development bias, while in the toner save mode, Among the correlation data belonging to the low density side correlation data, the exposure energy is higher than the optimum exposure energy in the normal mode, and the absolute value of the development bias is lower than the absolute value of the optimum development bias in the normal mode. An image forming method characterized in that the correlation data is set as an optimum exposure energy and an optimum development bias.

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