JP3853909B2 - Image forming apparatus - Google Patents

Description

と表すことができる。ここで、Ｂは、Ａの湾曲度を変えるための関数である。
【００２８】
この関数の一例としては、８ビット画像信号の場合、０＝Ｂ（０，ｎ），２５５＝Ｂ（２５５，ｎ）（ｎは任意の整数）を満たす２次のベジエ関数を用いる。上記の条件を満たすベジエ関数は、始点Ｐ０（０，０）と終点Ｐ１（２５５，２５５）とを結ぶ直線Ｐ０Ｐ１と直線Ｐ０Ｐ１を交わる直線Ｌと、その直線Ｌ上に存在し、直線Ｐ０Ｐ１と直線Ｌとの交点からの距離ｄをパラメータとする制御点Ｐ２とから２次のベジエ曲線として表される。
上記の関数では、関数Ｂの引数である整数ｃｕｒｖａｔｕｒｅに応じて距離ｄを比例させることにより、湾曲度を変えることができる。
例として、直線Ｐ０Ｐ１と直交する直線Ｌ１に対する場合と、図の縦軸に平行な直線Ｌ２に対する例について述べる。
第１の例における制御点を、両端点Ｐ０、Ｐ１の作る線分Ｐ０Ｐ１の中心点ＰＣ＝（Ｐ０＋Ｐ１）／２＝（１２７．５，１２７．５）または、（１２７，１２７）ｏｒ（１２８，１２８）に対し、この点に対する距離ｄをパラメータとしたとき、制御点Ｐ２は、
Ｐ２（ｄ）＝ＰＣ＋（−ｄ／√２，ｄ／√２）＝（１２７．５−ｄ／√２，１２７．５＋ｄ／√２）で与えられる（図８）。これにより、階調変換曲線Ｐ（ｄ，ｔ）は、
Ｐ（ｄ，ｔ）＝Ｐ０・ｔ２＋２・Ｐ２（ｄ）・ｔ・（１−ｔ）＋Ｐ１・（１−ｔ）２ （１）
で与えられる。
【００２９】
ただし、ｔは、０≦ｔ≦１の媒介変数である。Ｐ（ｄ，ｔ）は、階調変換曲線への入力ｘと出力ｙの組（ｘ，ｙ）として与えられるので、関数Ｂ（）への引数として与えられた整数Ａからｘ＝Ａとして、式１からｔを求め、求められたｔを再度式（１）に代入し、出力値ｙを求める。
実際には、上記のような計算を毎回行う代わりに、予め（ｘ，ｙ）の全ての組（０≦ｘ≦２５５）について求め、それをテーブルとして、ＲＯＭ中に記憶させておくことにより、計算時間を省略することができる。この階調補正テーブルを湾曲度を変えて数組（あるいは数１０組）ＲＯＭ３１中に保持する。湾曲度は、前述した関数Ｂ（）への引数ｃｕｒｖａｔｕｒｅで与えられる。
これにより、〈リスト１〉は、次のように書き換えられる。
【００３０】

なお、上記の例では、ｔａｂｌｅ＿ｍａｘ＝９としていることから、湾曲度が異なるテーブルの本数を９本としている。
なお、上記の例では、ベジエ曲線を用いたが、他にも、必要に応じて高次関数や指数・対数関数などを用いることもできる。
【００３１】
次に、図６の（ＳＴＥＰ２０２）および（ＳＴＥＰ２０３）の処理について説明する。
上記と同様にして、低画像濃度（ハイライト）領域、高画像濃度（シャドー）領域の湾曲度を変えることができる。
〈リスト２〉をより一般的な形に書き直すと次のようになる：

ここで、ハイライト変換曲線ＣＨ［ｈ］、シャドー変換曲線ＣＳ［ｓ］の変換を実行すると、

と表すこともできる。この中で、ｃｕｒｖａｔｕｒｅ，ｈ，ｓは、それぞれ全体、ハイライト、シャドー部の湾曲度を決める値である。
【００３２】
なお、ハイライト部と、シャドー部の湾曲は、互いに独立に作成されている。
ハイライト領域、およびシャドー領域のように、特定の濃度領域の湾曲度を変えるための階調変換曲線を次のように生成する。
始点Ｐ０と終点Ｐ１とを結ぶＰ０Ｐ１と直線Ｐ０Ｐ１を交わる直線Ｌと、その直線Ｌ上に存在し、直線Ｐ０Ｐ１と直線Ｌとの交点からの距離ｄをパラメータとする制御点Ｐ２とから３次のベジエ曲線を用いて階調変換曲線を生成する。
ここでは、一例として、直線Ｐ０Ｐ１と直交する直線Ｌ１に対する場合と、図の縦軸に平行な直線Ｌ２に対する実施例について述べる。
ハイライト領域の階調特性を変える変換曲線は、図９に示す様に、一例として次のように生成する。始点Ｐ０、終点Ｐ１をそれぞれＰ０＝（０，０）、Ｐ１＝（２５５，２５５）とし、第１の制御点Ｐ２をＰ２＝（３２，３２）とする。
第１の例における制御点Ｐ３は、直線Ｐ０Ｐ１と直線Ｌ１との交点からの距離ｄをパラメータとして、Ｐ３（ｄ）＝（１６，１６）＋（−ｄ／√２，ｄ／√２）とする。
【００３３】
第２の例における制御点Ｐ３は、直線Ｐ０Ｐ１と直線Ｌ２との交点からの距離ｄをパラメータとして、Ｐ３（ｄ）＝（１６，１６）＋（０，ｄ）とする。
以上のＰ０〜Ｐ３を用いて、階調変換曲線Ｐ（ｄ，ｔ）は、
Ｐ（ｄ，ｔ）＝Ｐ０・ｔ３＋３・Ｐ２・ｔ２・（１−ｔ）＋３・Ｐ３（ｄ）・ｔ（１−ｔ）２＋Ｐ１・（１−ｔ）３ （２）
で与えられる。
ここでは、終点として、Ｐ１＝（２５５，２５５）としたが、終点Ｐ１をＰ１＝（６４，６４）など、線分ｍ：（０，０）−（２５５，２５５）上の点とする。このとき、線分ｍ上で線分Ｐ０Ｐ１に含まれない線分は階調変換としてそのまま恒等変換として用い、それ以外の領域が、ハイライト領域、およびシャドー領域のように、特定の濃度領域の湾曲度を変えるための階調変換曲線として作用する。
【００３４】
次に、画像濃度（階調性）の自動階調補正（ＡＣＣ：Ａｕｔｏ Ｃｏｌｏｒ
Ｃａｌｉｂｒａｔｉｏｎ）の動作を図１０のフローチャートに基づいて説明する。
操作部（図１１）の液晶画面１５６において、ＡＣＣメニューを呼び出すと、図１２に示す画面が表示される。コピー使用時、あるいはプリンター使用時用の自動階調補正の［実行］を選択すると、図１３に示す画面が表示される。コピー使用時を選択した場合には、コピー使用時に使用する階調補正テーブルが、プリンター使用時を選択するとプリンター使用時の階調補正テーブルが参照データに基づいて変更される。
ここで、印刷スタートキーを選択すると、図１４に示すような、ＹＭＣＫ各色、および文字、写真の各画質モードに対応した、複数の濃度階調パターンを転写材上に形成する（ＳＴＥＰ２０５、２０６、２０７）。この濃度階調パターンは、あらかじめＩＰＵのＲＯＭ３１中に記憶・設定がなされている。パターンの書き込み値は、１６進数表示で、００ｈ，１１ｈ，２２ｈ，…，ＥＥｈ，ＦＦｈの１６パターンである。図では、地肌部を除いて５階調分のパッチを表示しているが、００ｈ−ＦＦｈの８ビット信号の内、任意の値を選択することができる。文字モードでは、パターン処理などのディザ処理を行わず、１ビット２５６階調でパターンが形成され、写真モードでは、主走査方向に隣接し２画素ずつの書き込み値の和を配分してレーザーの書き込み値が形成される。すなわち、
１画素目の画素の書き込み値がｎ１、２画素目の書き込み値がｎ２である場合のパターン処理は、
ｎ１＋ｎ２≦２５５の場合、
１画素目の書き込み値：ｎ１＋ｎ２、２画素目の書き込み値：０
ｎ１＋ｎ２＞２５５の場合、
１画素目の書き込み値：２５５、２画素目の書き込み値：ｎ１＋ｎ２−２５５または、
ｎ１＋ｎ２≦１２８の場合、
１画素目の書き込み値：ｎ１＋ｎ２、２画素目の書き込み値：０
１２８＜ｎ１＋ｎ２≦２５６の場合、
１画素目の書き込み値：１２８、２画素目の書き込み値：ｎ１＋ｎ２−１２８２５６＜ｎ１＋ｎ２≦３８３の場合、
１画素目の書き込み値：ｎ１＋ｎ２−１２８、２画素目の書き込み値：１２８３８３＜ｎ１＋ｎ２の場合、
１画素目の書き込み値：２５５、２画素目の書き込み値：ｎ１＋ｎ２−２５５等と配分する。これ以外にも、実際に画像形成時に使用しているパターン処理を用いる。
【００３５】
転写材にパターンが出力された後、転写材を原稿台１１８上に載置するように、操作画面上には、図１５に示す画面が表示される。
パターンが形成された転写材を原稿台に載置し（図１０のＳＴＥＰ２０８）、読み取りスタートを選択すると、スキャナーが走行し、ＹＭＣＫ濃度パターンのＲＧＢデータを読み取る（ＳＴＥＰ２０９）。この際、パターン部のデータと転写材の地肌部のデータを読み取る。パターンの読み取り値を、後で詳述するＲＧＢ補正値を用いて補正する（ＳＴＥＰ２１０、２１１）。
地肌データを用いた処理を行うと選択された場合（ＳＴＥＰ２１２）には、読み取りデータに対する地肌データ処理を行い（ＳＴＥＰ２１３）、参照データの補正を行う場合（ＳＴＥＰ２１４）には、参照データに対する高画像濃度部の処理（ＳＴＥＰ２１５）を行った後、ＹＭＣＫ階調補正テーブルを作成・選択を行う（ＳＴＥＰ２１６）。
上記の処理をＹＭＣＫの各色（ＳＴＥＰ２１７）、および写真、文字の各画質モード毎に行う（ＳＴＥＰ２１８）。処理中には、操作画像には図１６の示す画面が表示される。
処理終了後のＹＭＣＫ階調補正テーブルで画像形成を行った結果が、望ましくない場合には、処理前のＹＭＣＫ階調補正テーブルを選択することができるように、［元に戻す］キーが図１２に示す画面中に表示されている。
【００３６】
次に、地肌の補正について説明する。
地肌の補正の処理の目的として２つある。１つは、ＡＣＣ時に使用される転写材の白色度を補正することである。これは、同一の機械に、同じ時に画像を形成しても、使用する転写材の白色度によって、スキャナーで読み取られる値が異なるためである。補正しない場合のデメリットとしては、例えば、白色度が低い、再生紙などをこのＡＣＣに用いた場合、再生紙は一般にイエロー成分が多いために、イエローの階調補正テーブルを作成した場合に、イエロー成分が少なくなるように補正する。この状態で、次に、白色度が高いアート紙などでコピーをした場合に、イエロー成分が少ない画像となって望ましい色再現が得られない場合がある。
もう一つの理由としては、ＡＣＣ時に用いた転写紙の厚さ（紙厚）が薄い場合には、転写材を押えつける圧板など色が透けてスキャナーに読み取られてしまう。例えば、圧板の代わりにＡＤＦ（Ａｕｔｏ Ｄｏｃｕｍｅｎｔ Ｆｅｅｄｅｒ）と呼ばれる原稿自動送り装置を装着している場合には、原稿の搬送用にベルトを用いているが、これが使用しているゴム系の材質により、白色度が低く、若干の灰色味がある。そのため、読み取られた画像信号も、見かけ上、全体に高くなった画像信号として読み取られるために、ＹＭＣＫ階調補正テーブルを作成する際に、その分薄くなるように作成する。この状態で、今度は紙厚が厚く、透過性が悪い転写紙を用いた場合には、全体の濃度が薄い画像として再現されるため、必ずしも望ましい画像が得られない。
【００３７】
上記のような不具合を防ぐために、紙の地肌の読み取り画像信号から紙の地肌部の画像信号により、パターン部の読み取り画像信号の補正を行っている。
しかし、上記の補正を行わない場合にもメリットがあり、常に再生紙のようにイエロー成分が多い転写紙を用いる場合には、補正をしない方がイエロー成分が入った色に対しては色再現が良くなる場合がある。また、常に、紙厚が、薄い転写紙のみしか用いない場合には、薄い紙に合わせた状態に階調補正テーブルが作成されるというメリットがある。
上記のように、使用者の状況と好みとに応じて、地肌部の補正をＯＮ／ＯＦＦすることができる。
【００３８】
次に、画像形成用プリンタγ補正回路１１２に設定する階調変換テーブルの作成方法について述べる。まず初めに、現像特性の検知方法について図１７のフローチャートを用いて説明する。ＳＴＥＰ２１９では、図１８に図示するように、感光体２上にｎｐ個（ここでは、ｎｐ＝１２）の濃度階調パターンを形成する。表面電位センサー３９で感光体の表面電位Ｖｓｉ（ｉ＝１，２，…，ｎｐ）を読み込み（ＳＴＥＰ２２０）、現像器により現像することにより、顕像化される（ＳＴＥＰ２２１）。次に、感光体２の回転方向下流側に存在する光学センサー３６により、感光体２上のトナー像の検知出力Ｖｐｉ（ｉ＝１，２，…，ｎｐ）を得る（ＳＴＥＰ２２２）。検知に用いるレーザー出力は、一例として、画像信号の値（１６進数表示）で００（Ｈ）、１０（Ｈ）、２０（Ｈ）、３０（Ｈ）、４０（Ｈ）、５０（Ｈ）、６０（Ｈ）、７０（Ｈ）、９０（Ｈ）、Ｂ０（Ｈ）、Ｄ０（Ｈ）、ＦＦ（Ｈ）を用いる。主走査方向の２画素ずつの画像信号の和をその値に応じて次のように２画素に割り振る。すなわち、１画素目の画像信号をＮ₁、２画素目の画像信号をＮ₂、処理後の１画素目の画像信号をＮ₁’、２画素目の画像信号をＮ₂’とすると、Ｎ₁＋Ｎ₂≦ＦＦ（Ｈ）のとき、Ｎ₁’＝Ｎ₁＋Ｎ₂Ｎ₂’＝０Ｎ₁＋Ｎ₂＞ＦＦ（Ｈ）のとき、Ｎ₁’＝ＦＦ（Ｈ）
Ｎ₂’＝Ｎ₁＋Ｎ₂−ＦＦ（Ｈ）
次に、画像信号の補正方法について図１９を用いて説明する。グラフａ）の縦軸は、レーザー出力（または画像出力信号）、横軸は、光学センサー３６の出力を表す。このグラフは、ｎｐ個の濃度階調パターン潜像を感光体ドラム２上に形成した後、現像し、そのトナー像の反射光量を光学センサー３６で検知することによって得られる。グラフｂ）の縦軸は、ａ）と同じくレーザー出力、横軸は、感光体の表面電位を表す。これは、感光体の光減衰特性を表し、ａ）と同じく、ｎｐ個の濃度階調パターン潜像を感光体ドラム２上に形成したときの表面電位を電位センサーによって測定することにより得られる。グラフｃ）は、画像形成部に用いる階調変換テーブルを表し、図の横軸は、画像入力信号（これは、例えば原稿画像の濃度に比例する量）で、縦軸は、レーザーの出力または画像入力信号を階調変換テーブルによる変換を行った画像信号（画像出力信号）を表す。ここでは、画像入力信号は８ビット（２５６値）の分解能を有し、レーザーの書き込み光量も、同様にレーザーの最小値と最大値との間の８（〜１０）ビットの分解能を持つ。
【００３９】
図中で、ａは検知時に用いられるレーザー出力と画像入力信号との関係を表す。
グラフｄ）の縦軸は、感光体２上のトナー付着量、横軸は、光学センサー３６の出力を表し、これは、光学センサーの出力特性を表す。この特性は、使用するセンサーの種類や取付角度や感光体２からの距離などによって異なるが、これは予め知られており、ほぼ一定である。
グラフｅ）の縦軸は、トナー付着量、横軸は、感光体２の表面電位を表す。これは、感光体２の表面電位と感光体２上のトナー付着量の関係（すなわち、現像特性）を表す。
図中のｈは、現像バイアスのＤＣ成分を表す。
グラフｆ）は、画像入力信号に対する感光体２上のトナー付着量の関係を表す。
グラフｄ）の関係を用いて光学センサー３６の出力ＶＰｉを感光体２上トナー付着量（Ｍ／Ａ）ｉ［ｍｇ／ｃｍ２］（ｉ＝１，２，…，ｎｐ）に換算する。これは、一例として以下に述べる方法により求める。
【００４０】
感光体２上に形成されたトナー像の反射光は、光電センサー３６により検出され、検知信号としてメイン制御部３０に送られる。ＶＳＰ、ＶＳＧをそれぞれ基準パターン部のトナー付着部からの光電センサー出力および地肌部の出力として、基準パターンに付着したトナーの単位面積当りの付着量ｍ₁ ［ｇ／ｃｍ² ］は、
ｍ₁ ＝−１ｎ（ＶＳＰ／ＶＳＧ）／β
β＝−６．０×１０３［ｃｍ² ／ｇ］
の関係からトナー付着量が換算される。ここで、βは、光電センサーとトナーによって決まる定数であり、上記の値は黒トナーの値である。イエロー、シアン、マゼンタについても同様に換算することができる。ここでは、計算を行ったが、予め作成されたルックアップテーブルにより、変換してもよい。
上記の方法により、感光体表面電位ＶＳｉと感光体上トナー付着量（Ｍ／Ａ）ｉとの関係が求められ、グラフｅ）の現像特性ｉが得られる。
【００４１】
しかしながら、グラフｄ）に示すように、光学センサーの出力は、あるトナー付着量（Ｍ／Ａ）Ｃより高いトナー付着量領域（（Ｍ／Ａ）≧（Ｍ／Ａ）Ｃ）では、一定の値ＶＰＭＩＮを示す。一方、図中ｃ）のｎという画像入力信号以上の画像入力信号に対しては、実際には、ｂ）に示す様に感光体２の表面電位が低下し、トナー付着量が変化しているにも関わらず、感光体上トナー付着量（Ｍ／Ａ）は常に一定量（Ｍ／Ａ）Ｃになる。そのため、グラフｅ）中で、実際の現像特性がｃであっても、検知した結果から求めた現像特性はｉのようになり、実際の値ｃと検知された値ｉとの間でずれが生じる。
【００４２】
ここで、実際の現像特性と検知値から求めた現像特性のずれを補うために、次のような補正を行う（図２０参照）。
画像信号ｉに対する光学センサーの検出値ＶＰｉが、所定値ＶＰｃ以上である場合、その検出値から感光体２上のトナー付着量またはそれにほぼ比例する量（Ｍ／Ａ）ｉに換算する（図２０のＳＴＥＰ２２３）。これらの値から、表面電位センサーの出力値ＶＳｉと（Ｍ／Ａ）ｉとの関係式を求める。ここでは、１次式を用いて次のような関係
（Ｍ／Ａ）ｉ＝ａ×ＶＳｉ＋ｂ （ＶＰｉ≧ＶＰｃ） （２）
もしくは、現像バイアスのＤＣ成分をＶＤＣとして、
（Ｍ／Ａ）ｉ＝ａ×（ＶＳｉ−ＶＤＣ）＋ｂ （ＶＰｉ≧ＶＰｃ）
を用いる。ここで、ａ，ｂは係数で、ＶＳｉと（Ｍ／Ａ）ｉの値から最小自乗法等の方法を用いて決定する（ＳＴＥＰ２２４、２２５）。ここで、光学センサー３６の出力値がＶＰｃとなる感光体２上のトナー付着量を（Ｍ／Ａ）Ｃとすると、（Ｍ／Ａ）ｉ≦（Ｍ／Ａ）Ｃを満たす付着量範囲としても同じである。
【００４３】
感光体２上のトナー付着量がある値（Ｍ／Ａ）ＭＩＮより低い領域では、トナー付着量と感光体２上の表面電位との直線関係からのずれが大きくなる場合がある。それを防ぐために、（Ｍ／Ａ）ＭＩＮ≦（Ｍ／Ａ）≦（Ｍ／Ａ）Ｃを満たす感光体上トナー付着量の検知結果について、前述した式（２）の係数ａ，ｂを決定する（ＳＴＥＰ２２６）。
ここでは、トナー付着量を用いたが、（Ｍ／Ａ）ＭＩＮに対応する光学センサー３６の検知出力をＶＰＭＡＸとして
ＶＰｃ≦ＶＰ≦ＶＰＭＡＸ
を満たすトナー付着領域に対応するトナー付着領域から前述した式（２）の係数ａ，ｂを決定してもよい（ＳＴＥＰ２２７、２２８）。
以上のフローチャートを図２０に示した。
【００４４】
上記の例では、所定値ＶＰｃに対する光学センサー３６の検知出力の大小関係からその値を用いるかどうかを判断したが、このＶＰｃは以下のように、感光体２のトナー付着量（Ｍ／Ａ）の変化△（Ｍ／Ａ）に対する、光検知手段の出力値ＶＰの変化量△ＶＰの絶対値｜△ＶＰ／△（Ｍ／Ａ）｜が、所定値｜△ＶＰ／△（Ｍ／Ａ）｜₀ と等しい感光体上トナー付着量を（Ｍ／Ａ）Ｃ、そのときの光学センサーの出力値をＶＰｃとすることにより、前述した方法を用いることができる。この場合、
｜△ＶＰ／△（Ｍ／Ａ）｜≧｜△ＶＰ／△（Ｍ／Ａ）｜₀ を満たす感光体上トナー付着量領域が、
（Ｍ／Ａ）≦（Ｍ／Ａ）Ｃと対応し、｜△ＶＰ／△（Ｍ／Ａ）｜＞｜△ＶＰ／△（Ｍ／Ａ）｜₀ が、
（Ｍ／Ａ）ｉ≦（Ｍ／Ａ）Ｃに対応する（図２１）。
このようにして求められた現像特性ｉと画像信号からグラフｆ）の検知結果ｃが得られる。
第１象現のａの階調変換テーブルに対して、第４象現の検知特性ｃが得られた。この結果を用いて、理想特性ｄを得るための補正後の階調変換テーブルｂ）を得ることができる。
上記のようにして得られた階調変換テーブルを画像形成用階調変換テーブルとして使用する。
【００４５】
以上のフローチャートを図２２に示した。
図２２において、（ＳＴＥＰ２２９）感光体上に基準パターンを作成する。
（ＳＴＥＰ２３０）形成された基準パターンを光電センサー、表面電位センサーにより検知する。
（ＳＴＥＰ２３１）検知された結果とパターン形成時に使用したレーザーの出力値（画像信号）から、現像特性を予測する。
（ＳＴＥＰ２３２）予測された現像特性などから画像形成部用の階調変換テーブルを作成する。
次に、現像特性が初期設定時から所定時間後、経時変化した場合の補正方法を述べる。
設定時の画像入力信号とレーザー出力との関係をａ、そのときのレーザー出力に対する光学センサーの検知データをｂ、前述した方法によって求められた感光体２の表面電位に対する感光体２上のトナー付着量の関係をｃ、また、画像入力画像信号と感光体２上のトナー付着量との関係をｄとする。
所定時間後の光学センサー３６の検知出力がｂ’となったとすると、これは、現像特性がｃからｃ’に変化したことを表し、その結果、画像入力画像信号に対する感光体２上のトナー付着量の関係はｄ’となったことを表す。そのため、初期と所定時間後では、階調性は変化してしまっている。
初期と所定時間後で階調性が変化してしまっていることが、画像再現の点から望ましいことではない。そのため、これを次のように補正する。
第１象現の画像入力信号ｎとレーザー出力Ｐとの関係は、初期設定時には、ａに示すように、線形の関係であるとする。すなわち、画像信号ＦＦＨの時のレーザー光量をＰＭＡＸとして、
Ｐ＝ＰＭＡＸ／ＦＦ（Ｈ）×ｎ
である。
【００４６】
画像入力信号とレーザー出力との関係がａであるとき、画像入力信号ｉに対するレーザー出力はＰｉになり、感光体２上のトナー付着量は、初期（Ｍ／Ａ）ｉ（０）から所定時間後、（Ｍ／Ａ）ｉ（ｔ）に変化している。一方、画像入力信号ｊに対して、レーザー出力はＰｊで、このレーザー出力に対する感光体２上のトナー付着量は、初期（Ｍ／Ａ）ｊ（０）から所定時間後、（Ｍ／Ａ）ｊ（ｔ）に変化している。
上記の場合の内、特に（Ｍ／Ａ）ｉ（０）＝（Ｍ／Ａ）ｊ（ｔ）である場合、所定時間後の画像入力信号ｉとレーザー出力との関係を、ｉ→Ｐｉからｉ→Ｐｊと変更することにより、見かけ上、画像入力信号に対する画像濃度が初期と所定時間後で変化していないことになる。
このようにして、ｎｐ個の検知データを用いて、同様な処理を行うことにより、特性ａ’を求めることができる。この際、実際の測定点の間の値は、直線補間を行うか、スプライン曲線などにより、補間することにより、与えることができる。
あるいは、全ての点について計算を行わずに、検知されたｎｐ個のデータ点または、その中の何点かについて、上記の計算を行い、その値を用いてＲＯＭ３１中に記憶されたルックアップテーブルを選択し、それを補正特性ａ’として用いてもよい。
これにより、初期設定時と所定時間後の経時変化した階調性を見かけ上変化していないように補正することができる。
【００４７】
以上のフローチャートを図２３に示した。
すなわち、図２３のＳＴＥＰ２３３において現像特性の検知を行い、ＳＴＥＰ２３４において検知出力を（Ｍ／Ａ）ｉ（ｔ）として記憶し、ＳＴＥＰ２３５において現像特性を補正する。
上記の方法によって得られた所定時間後の特性ａ’において、画像入力信号ＦＦＨに対応するレーザー出力をＰＭＡＸ（ｔ）、初期設定時の画像入力信号ＦＦＨのレーザー出力をＰＭＡＸ（０）とした場合、ＰＭＡＸ（０）≠ＰＭＡＸ（ｔ）である時の補正方法として、画像信号００Ｈでのレーザー出力Ｐ００とＰＭＡＸ（０）との間の分解能を維持したままで、ＰＭＡＸ（ｔ）＝Ｐｋ（０）となる画像入力信号ｋまでを使用する場合と、ＰＭＡＸ（ｔ）との間を８〜１０ビットの分解能を持たせる場合の２つの補正方法がある。本発明では、上記の２つのいずれの方法を用いてもよいが、前者の場合は、レーザー光量の最大値を変更しないため、作像条件の制御が簡単になるが、実質的な階調数が減ってしまうという欠点がある。
【００４８】
上記の補正方法の他に、図２４に示すように、作像条件の１つである現像バイアスをｅ→ｅ’と変更することにより、現像開始電位（ここでは、付着量（Ｍ／Ａ）ＭＩＮでの表面電位）をｆ’→ｆ″と変化させることができる。この結果、感光体２上の表面電位に対する感光体上トナー付着量の関係がＣ’→Ｃ″と変わる。ただし、ここではｅ’は、初期の現像開始電位ｅと一致するように選択した。
この場合、画像入力信号とレーザー出力の間の補正量（ａとａ’との差とａとａ″との差）がａ→ａ’（図１９）に対して、ａ’→ａ″（図２４）は少なくて済む。補正テーブルを選択する方法の場合には、この方法を用いることにより補正幅が少なくて済むため、補正テーブルの記憶容量が少なくて済むというメリットがある。
現像バイアス他にも、感光体２の帯電電位などを制御することにより、同様な効果を得ることができる。この場合のフローチャートを図２５に示した。
すなわち、図２５のＳＴＥＰ２３６において現像特性の検知を行い、ＳＴＥＰ２３７において検知出力を（Ｍ／Ａ）ｉ（ｔ）として記憶し、ＳＴＥＰ２３８において作像条件を変更し、ＳＴＥＰ２３９において現像特性を補正する。
【００４９】
次に、画像処理用プリンタγ補正回路１０９に設定する階調補正テーブルの作成方法について説明する。パターン発生回路１２１で発生させた階調パターンの書き込み値をＬＤ［ｉ］（ｉ＝０，１，…，９）、形成されたパターンのスキャナーでの読み取り値（ｒ［ｔ］［ｉ］，ｇ［ｔ］［ｉ］，ｂ［ｔ］［ｉ］）（ｔ＝Ｙ，Ｍ，Ｃ，ｏｒｋ，ｉ＝０，１，…，９）とする。なお、階調パターン形成時には、画像形成部用の階調変換テーブルには、前述した方法によって得られた階調変換テーブルを設定しておく。（ｒ，ｇ，ｂ）の代わりに、明度、彩度、色相角（Ｌ＊，ｃ＊，ｈ＊）、あるいは、明度、赤み、青み（Ｌ＊，ａ＊，ｂ＊）などで表しても良い。参照データは、階調変換テーブルへの入力値ｎ（ｎ＝０，１，２，…，２５５）とスキャナーの読み取り値（ｒ［ｔ］［ｉ］，ｇ［ｔ］［ｉ］，ｂ［ｔ］［ｉ］）の目標値である。
【００５０】
参照データを以下のように表す：
式３
Ａｒ［ｔ］［ｎ］（０≦ｎ≦２５５，ｔ＝Ｙ，Ｍ，Ｃ，ｏｒ ｋ）
Ａｇ［ｔ］［ｎ］（０≦ｎ≦２５５，ｔ＝Ｙ，Ｍ，Ｃ，ｏｒ ｋ）
Ａｂ［ｔ］［ｎ］（０≦ｎ≦２５５，ｔ＝Ｙ，Ｍ，Ｃ，ｏｒ ｋ）
ここで、Ａｒ，Ａｇ，Ａｂは、それぞれＲｅｄ信号、Ｇｒｅｅｎ信号、Ｂｌｕｅ信号に対する参照データで、ＹＭＣＫは、トナーの色を表す。
上記式３は、８ビット信号処理で、階調変換テーブルへの入力値の取りうる値、すなわち０から２５５値までの２５６値に対応する参照データを、メモリ中に保持していることを表している。
上記のように２５６個の参照データをメモリ中に記憶しておくことにより、あとで述べる処理を簡単にすることができるが、参照データを記憶するためのメモリの量を節約するために、ｎ［０］＝０，ｎ［ｉ］＝２６×Ｉ−５（Ｉ＝１，２，…，１０）を一例としたいくつかのｎ［ｉ］（この場合には、１６個）の値と対応する参照データ（式３）との組
式４
ｎ［ｉ］（０≦ｎ［ｉ］≦２５５，ｉ＝０，１，２，…，１０）
Ａｒ［ｔ］［ｎ［ｉ］］（０≦ｎ［ｉ］≦２５５，ｉ＝０，１，２，…，１０，ｔ＝Ｙ，Ｍ，Ｃ，ｏｒ ｋ）
Ａｇ［ｔ］［ｎ［ｉ］］（０≦ｎ［ｉ］≦２５５，ｉ＝０，１，２，…，１０，ｔ＝Ｙ，Ｍ，Ｃ，ｏｒ ｋ）
Ａｂ［ｔ］［ｎ［ｉ］］（０≦ｎ［ｉ］≦２５５，ｉ＝０，１，２，…，１０，ｔ＝Ｙ，Ｍ，Ｃ，ｏｒ ｋ）
をメモリ中に記憶して、ｎ［ｉ］（Ｉ＝０，１，２，…，１０）以外のｎ（上記の例では、ｎ＝１〜２０など）に対する参照データＡｒ［ｔ］［ｎ［ｉ］］などは、後述するように、補間を行うことによって算出しても良い。一例として、ｎ［ｉ］≦ｎ≦ｎ［ｉ＋１］となるｎ［ｉ］，ｎ［ｉ＋１］（ｎ＝１〜２０に対しては、Ｉ＝０，ｎ［０］＝０，ｎ［１］＝２１）に対応する参照データＡｒ，ｇ，ｂ［ｔ］［ｎ［ｉ］］，Ａｒ，ｇ，ｂ［ｔ］［ｎ［ｉ＋１］］を用いて補間を行うことにより求める。
【００５１】
一方、ＲＡＭ３２中には、ＹＭＣＫトナーのそれぞれに対し、パターンの読み取り値の参照データにおけるＲＧＢ成分の大きさの割合、
Ｋ［ｓ］［ｔ］｛ｓ＝Ｒ，Ｇ，ｏｒ Ｂ； ｔ＝Ｙ，Ｍ，Ｃ，ｏｒ Ｋ｝
が記憶されている。Ｋ［ｓ］［ｔ］は、１付近の値をとる。ただし、複写機内部では以下のように、整数データとして保持している。
Ｋ［ｓ］［ｔ］＝Ｋ１［ｓ］［ｔ］／２ⁿ （Ａ Ｋ１［ｓ］［ｔ］は、整数）例えば、ｎ＝１０、２＾ｎ（２のｎ乗を意味する）＝１０２４などである。
【００５２】
ＲＧＢ信号の補正値であるＫ［ｓ］［ｔ］の値の一例を図２６に示す。
図２６に示されたＲＧＢ信号の補正データは、図２７に示すように、複写機本体１の操作部の表示画面１５６に表示され、表示箇所の該当する部分を指で押圧することによりそれら数値の入力ができる。入力されたデータはＲＡＭ３２内に記憶される。
一例として、ｔ＝Ｃ（シアン）の場合について説明する。シアントナーの参照データのＲＧＢ成分を、
式５
Ａｒ１［Ｃ］［ｎ［ｉ］］＝Ａｒ［Ｗ］＋（Ａｒ［Ｃ］［ｎ［ｉ］］−Ａｒ［Ｗ］×ｋ［ｒ］［Ｃ］
Ａｇ１［Ｃ］［ｎ［ｉ］］＝Ａｇ［Ｗ］＋（Ａｒ［Ｃ］［ｎ［ｉ］］−Ａｒ［Ｗ］×ｋ［ｇ］［Ｃ］
Ａｂ１［Ｃ］［ｎ［ｉ］］＝Ａｂ［Ｗ］＋（Ａｒ［Ｃ］［ｎ［ｉ］］−Ａｒ［Ｗ］×ｋ［ｂ］［Ｃ］
（Ｉ＝０，１，２，…，１０）
と補正する。ここで、（Ａｒ１［Ｃ］［ｎ［Ｉ］］，Ａｇ１［Ｃ］［ｎ［ｉ］］，Ａｂ１［Ｃ］［ｎ［Ｉ］］）は、それぞれ、補正後の参照データのＲＧＢ成分を表し、（Ａｒ［ｔ］［ｎ［Ｉ］］，Ａｇ［ｔ］［ｎ［ｉ］］，Ａｂ［ｔ］［ｎ［ｉ］］）は、補正前の参照データである。
【００５３】
Ａｒ［Ｗ］，Ａｇ［Ｗ］，Ａｂ［Ｗ］は、それぞれ、白色（使用するスキャナーにとって最も明るい色）を読み取った時のＲＧＢ信号である。この値は、読み取り値が８ビット信号である場合には、０から２５５値の範囲にあり、０値は最も暗い画像濃度（反射率、または透過率が低い物体を読み取ったときのスキャナーのＣＣＤが検知する光量）、２５５値は最も明るい画像濃度（反射率、または透過率が高い物体を読み取った時の値スキャナーのＣＣＤが検知する光量）、２５５値近辺の値を有する。
上記の場合より、若干精度が低下するが、実使用上は、
Ａｒ［Ｗ］＝Ａｒ［Ｃ］［０］，Ａｇ［Ｗ］＝Ａｇ［Ｃ］［０］，Ａｂ［０］＝Ａｂ［Ｃ］［０］
としてもよい。
ここで、Ａｒ［Ｃ］［０］、Ａｇ［Ｃ］［０］、Ａｂ［Ｃ］［０］は、紙の地肌部を読み取った値である。
ここで、紙の地肌部を読み取る際には、紙の裏面に、紙を数枚重ね（いわゆる、ホワイトバック）紙の裏当てが暗くならないように注意することにより、精度が低下することを防ぐことができる。
【００５４】
別の実施例として、同様に、ｔ＝Ｃ（シアン）の場合の例を示すと、
式６

として、処理することも実用上は可能である。
ただし、Ｉ＝０，ｎ［０］＝０，すなわち、階調補正テーブルへの入力値が０の場合に、式６の補正を行わないようにしている。
式６で用いるｋ［ｒ］［Ｃ］，ｋ［ｇ］［Ｃ］，ｋ［ｂ］［Ｃ］の値と、式５で用いたｋ［ｒ］［Ｃ］，ｋ［ｇ］［Ｃ］，ｋ［ｂ］［Ｃ］とは、同一の数値ではなく、使用する式によって数値を適正な値に変更する必要がある。
なお、処理を簡単にするために、上式の、（Ａｒ１［Ｃ］［ｎ［Ｉ］］，Ａｇ１［Ｃ］［ｎ［ｉ］］，Ａｂ１［Ｃ］［ｎ［ｉ］］）を新たな（Ａｒ［ｔ］［ｎ［ｉ］］，Ａｇ［ｔ］［ｎ［ｉ］］，Ａｂ［ｔ］［ｎ［ｉ］］）として、以下で用いる。
【００５５】
次に、ＡＣＣ実行時に作成・選択され、画像処理用プリンタγ補正回路１０９に設定される階調変換テーブル（ＬＵＴ）の生成方法を説明する。
ＹＭＣ各トナーの補色の画像信号は、それぞれブルー、グリーン、レッドであるので、処理を簡単にするために、上記の参照データＡｒ［ｔ］［ｉ］，Ａｇ［ｔ］［ｉ］，Ａｂ［ｔ］［ｉ］のうち、各トナーに対するそれぞれの補色の参照データＡｂ［ｔ］［ｉ］，Ａｇ［ｔ］［ｉ］，Ａｒ［ｔ］［ｉ］を用いる。
上記の取り扱いは、使用するトナーの分光（反射率）特性が大きく変化しない場合（つまり色味が変わらない場合）に有効である。
後の記載を簡単にするために、Ａ［ｔ］［ｎ［ｉ］］（０≦ｎ［ｉ］≦２５５；ｉ＝１，２，…，１０；ｔ＝Ｃ，Ｍ，Ｙ）を用いて表す。なお、ブラックトナーについては、ＲＧＢのいずれの画像信号を用いても十分な精度が得られるが、ここでは、Ｇ（グリーン）成分を用いる。
同様に、読み取り信号も補色の画像信号のみを用いてａ［ｔ］［ｉ］（ｉ＝０，１，…，９；ｔ＝Ｃ，Ｍ，Ｙ，Ｋ）で表す。
【００５６】
また、ある色のトナーｔ（ｔ＝Ｃ，Ｍ，Ｙ，Ｋ）に対する、参照データＡ［ｔ］［ｉ］とＬＤの書き込み値ａ［ｔ］［ｉ］とを、以下では、Ａ［ｉ］とａ［ｉ］と略して表記した。
ＹＭＣＫ階調変換テーブルは、前述したａ［ＬＤ］とＡ［ｎ］とを比較することによって得られる。ここで、ｎは、ＹＭＣＫ階調変換テーブルへの入力値で、ＲＧＢ信号を補正した参照データＡ［ｎ］は、入力値ｎをＹＭＣＫ階調変換した後のレーザー書き込み値ＬＤ［ｉ］で出力したＹＭＣトナー・パターンを、スキャナーで読み取った読み取り画像信号の目標値である。ここで、ＲＧＢ信号を補正した参照データには、プリンターの出力可能な画像濃度に応じて補正を行う参照データと補正を行わない参照データの２種類がある。補正を行うかどうかの判断は、予めＲＯＭ３１またはＲＡＭ３２中に記憶されている後述する判断用のデータにより判断される。この補正については後述する。
前述した参照データＡ［ｎ］に対応するＬＤを求めることにより、ＹＭＣＫ階調変換テーブルへの入力値ｎに対応するレーザー出力ＬＤ［ｎ］を求める。
【００５７】
これを、入力値ｉ＝０，１，…，２５５（８ ｂｉｔ信号の場合）に対して求めることにより、階調変換テーブルを求めることができる。
その際、ＹＭＣＫ階調変換テーブルに対する入力値ｎ＝００ｈ，０１ｈ…，ＦＦｈ（１６進数）に対するすべての値に対して、上記の処理を行う代わりに、ｎ［ｉ］＝０，１１ｈ，２２ｈ，…，ＦＦｈのようなとびとびの値について上記の処理を行い、それ以外の点については、スプライン関数などで補間を行うか、あるいは、予めＲＯＭ３１中に記憶されているＹＭＣＫ階調変換テーブルの内の上記の処理で求めた（０，ＬＤ［０］），（１１ｈ，ＬＤ［１１ｈ］），（２２ｈ，ＬＤ［２２ｈ］），…，（ＦＦｈ，ＬＤ［ＦＦｈ］）の組を通る、最も近いテーブルを選択する。
【００５８】
上記の処理を図２８に基づいて説明すると、図の第１象現（ａ）の横軸は、ＹＭＣＫ階調変換テーブルへの入力値ｎ、縦軸は、スキャナーの読み取り値（処理後）で、前述した参照データＡ［ｉ］を表す。スキャナーの読み取り値（処理後）は、階調パターンをスキャナーで読み取った値に対し、ＲＧＢγ変換（ここでは変換を行っていない）、階調パターン内の数ケ所の読み取りデータの平均処理および加算処理後の値であり、演算精度向上のために、ここでは、１２ビットデータ信号として処理する。
図の第２象現（ｂ）の横軸は、縦軸と同じく、スキャナーの読み取り値（処理後）を表す。
【００５９】
第３象現（ｃ）の縦軸は、レーザー光（ＬＤ）の書き込み値を表す。このデータａ［ＬＤ］は、プリンター部の特性を表す。また、実際に形成するパターンのＬＤの書き込み値は、００ｈ（地肌），１１ｈ，２２ｈ，…，ＥＥｈ，ＦＦｈの１６点であり、とびとびの値を示すが、ここでは、検知点の間を補間し、連続的なグラフとして扱う。
第４象現のグラフ（ｄ）は、ＹＭＣＫ階調変換テーブルＬＤ［ｉ］で、このテーブルを求めることが目的である。
グラフ（ｆ）の縦軸・横軸は、グラフ（ｄ）の縦軸・横軸と同じである。検知用の階調パターンを形成する場合には、グラフ（ｆ）に示したＹＭＣＫ階調変換テーブル（ｇ）を用いる。
グラフ（ｅ）の横軸は、第３象現（ｃ）と同じであり、階調パターン作成時のＬＤの書き込み値と階調パターンのスキャナーの読み取り値（処理後）との関係を表すための、便宜上の線形変換を表す。
ある入力値ｎに対して参照データＡ［ｎ］が求められ、Ａ［ｎ］を得るためのＬＤ出力ＬＤ［ｎ］を階調パターンの読み取り値ａ［ＬＤ］を用いて、図中の矢印に沿って求める。
【００６０】
演算手順を図２９に基づいて説明する。図２９はＡＣＣ実行時の階調変換テーブルの作成手順を示すフローチャートである。
（ＳＴＥＰ２４０）ＹＭＣＫ階調変換テーブルを求めるために必要な入力値を決める。
ここでは、ｎ［ｉ］＝１１［ｈ］×ｉ（ｉ＝０，１，…，ｉｍａｘ＝１５）とした。
（ＳＴＥＰ２４１）前述した手順でＲＧＢ信号の補正値を用いて参照データを補正する。
（ＳＴＥＰ２４２）参照データＡ［ｎ］を、プリンターの出力可能な画像濃度に応じて補正を行う。
ここで、プリンターで作成可能な最大画像濃度を得られるレーザーの書き込み値を、ＦＦｈ（１６進数表示）であるとし、この時のパターンの読み取り値ｍ［ＦＦｈ］をｍｍａｘとする。低画像濃度側から中間画像濃度側にかけて補正を行わない参照データＡ［ｉ］（ｉ＝０，１，…，ｉ１）、高画像濃度側の補正を行わない参照データＡ［ｉ］（ｉ＝ｉ２＋１，…，ｉｍａｘ−１）（ｉ２≧ｉ１，ｉ２≦ｉｍａｘ−１）、補正を行う参照データＡ［ｉ］（ｉ＝ｉ１＋１，…，ｉ２）とする。
【００６１】
以下では、ＲＧＢ−γ変換を行わない、原稿反射率に比例した画像信号と仮定して、具体的な計算方法を述べる。補正を行わない参照データの内、高画像濃度部の最も画像濃度が低い参照データＡ［ｉ２＋１］と、低画像濃度部の最も画像濃度が高い参照データＡ［ｉ１］とから、そのデータの差△ｒｅｆを求める。すなわち、
△ｒｅｆ＝Ａ［ｉ１］−Ａ［ｉ２＋１］ （７）
ここで、反転処理であるＲＧＢγ変換を行わない反射率リニアあるいは明度リニアの場合には、△ｒｅｆ＞０である。
一方、プリンター部で作成可能な最大画像濃度を得られるパターンの読み取り値ｍｍａｘから、同様に差△ｄｅｔを求める。すなわち、
△ｄｅｔ＝Ａ［ｉ１］−ｍｍａｘ （８）
とする。
【００６２】
これにより、
（７）、（８）から、高濃度部の補正を行った参照データＡ［ｉ］（ｉ＝ｉ１＋１，…，ｉ２）を、

とする。
（ＳＴＥＰ２４３）（ＳＴＥＰ２４０）で求めたｎ［ｉ］に対応するスキャナーの読み取り画像信号ｍ［ｉ］を参照データＡ［ｎ］から求める。
実際には、とびとびのｎ［ｊ］に対応する参照データＡ［ｎ［ｊ］］（０≦ｎ［ｊ］≦２５５，ｊ＝０，１，…ｊｍａｘ、ｎ［ｊ］≦ｎ［ｋ］ｆｏｒ ｊ≦ｋ）を次のようにする。すなわち、ｎ［ｊ］≦ｎ［ｉ］＜ｎ［ｊ＋１］となるｊ（０≦ｊ≦ｊｍａｘ）を求める。
８ｂｉｔ画像信号の場合、ｎ［０］＝０，ｎ［ｊｍａｘ］＝２５５、ｎ［ｊｍａｘ＋１］＝ｎ［ｊｍａｘ］＋１、Ａ［ｊｍａｘ＋１］＝Ａ［ｊｍａｘ］として参照データを求めておくと計算が簡単になる。
【００６３】
上記のようにして求めたｊから、ｍ［ｉ］を次式から求める、
ｍ［ｉ］＝Ａ［ｊ］＋（Ａ［ｊ＋１］−Ａ［ｉ］）・（ｎ［ｉ］−ｎ［ｊ］）／（ｎ［ｊ＋１］−ｎ［ｊ］） （１０）
また、参照データの間隔は、ｎ［ｊ］はできるだけ小さい間隔である方が、最終的に求める階調変換テーブルの精度が高くなる。
ここでは、一次式により補間したが、高次関数やスプライン関数などで補間を行っても良い。その場合には、
ｍ［ｉ］＝ｆ（ｎ［ｉ］）
とする。ｋ次関数の場合には、
【００６４】
【数１】

などとする。
（ＳＴＥＰ２４４）（ＳＴＥＰ２４２）で求められたｍ［ｉ］を得るためのＬＤの書き込み値ＬＤ［ｉ］を（ＳＴＥＰ２４２）と同様な手順によって求める、（ｊ−＞ｋとする）
ＲＧＢγ変換を行っていない画像信号データを処理する場合には、ＬＤの値が大きくなるに応じて、ａ［ＬＤ］が小さくなる。すなわち、
ＬＤ［ｋ］＜ＬＤ［ｋ＋１］に対して、ａ［ＬＤ［ｋ］］≧ａ［ＬＤ［ｋ＋１］］
となる。
【００６５】
ここで、パターン形成時の値をＬＤ［ｋ］＝００ｈ，１１ｈ，２２ｈ，…，６６ｈ，８８ｈ，ＡＡｈ，ＦＦｈ，（ｋ＝０，１，…，９）の１０値とした。これは、トナー付着量が少ない画像濃度では、トナー付着量に対するスキャナーの読み取り値の変化が大きいため、パターンの書き込み値ＬＤ［ｋ］の間隔を密にし、トナー付着量が多い画像濃度では、トナー付着量に対するスキャナーの読み取り値の変化が小さいために、間隔を広げて読み込む。
これによるメリットとしては、ＬＤ［ｋ］＝００ｈ，１１ｈ，２２ｈ，…，ＥＥｈ，ＦＦｈ（計１６点）などとパターンの数を増やす場合に比べて、トナー消費を抑えられること、また、高画像濃度領域では、ＬＤ書き込み値に対する変化が少ないこと、感光体上の電位ムラ、トナーの付着ムラ、定着ムラ、電位ムラなどの影響で、読み取り値が逆転したりしやすいためＬＤ書き込み値の間隔を狭めても必ずしも精度の向上に有効ではないことなどから、上記のようなＬＤ書き込み値でパターンを形成した。
【００６６】
ａ［ＬＤ［ｋ］］≧ｍ［ｉ］＞ａ［ＬＤ［ｋ＋１］］となるＬＤ［ｋ］に対して、
ＬＤ［ｉ］＝ＬＤ［ｋ］＋（ＬＤ［ｋ＋１］−ＬＤ［ｋ］）・（ｍ［ｉ］−ａ［ＬＤ［ｋ］］）／（ａ［ＬＤ［ｋ＋１］］−ａ［ＬＤ［ｋ］］）
とする。
０≦ｋ≦ｋｍａｘ（ｋｍａｘ＞０）としたとき、ａ［ＬＤ［ｋｍａｘ］］＞ｍ［ｉ］の場合（参照データから求めた目標値の画像濃度が高い場合）には、
ＬＤ［ｉ］＝ＬＤ［ｋ］＋（ＬＤ［ｋｍａｘ］−ＬＤ［ｋｍａｘ−１］）・（ｍ［ｉ］−ａ［ＬＤ［ｋｍａｘ−１］］）／（ａ［ＬＤ［ｋｍａｘ］］−ａ［ＬＤ［ｋｍａｘ−１］］）
として、１次式で外挿を行うことによって予測する。
また、１次式のほか、対数を取るなどして他の方法で外挿を行っても良い。
これにより、ＹＭＣＫ階調変換テーブルへの入力値ｎ［ｉ］と出力値ＬＤ［ｉ］の組（ｎ［ｉ］，ＬＤ［ｉ］）（ｉ＝０，１，…，１５）が求められる。
（ＳＴＥＰ２４５）では求められた（ｎ［ｉ］，ＬＤ［ｉ］）（ｉ＝０，１，…，１５）を元に、スプライン関数などて内挿を行うか、あるいは、ＲＯＭ３１中に有している階調変換テーブルを選択する。
【００６７】
以下では、前述した補正階調曲線作成に関して、ＲＯＭ３１中に記憶されている階調変換テーブルの選択方法について図３０に基づいて説明する。
（ＳＴＥＰ２４６）では階調変換テーブル全体にかける係数ＩＤＭＡＸ［％］を求める。
ｎ［ｉｍａｘ］＝ＦＦｈの場合には、ＩＤＭＡＸ＝ＬＤ［ｉｍａｘ］／ＦＦｈ×１００［％］とする。
ここでは、ＬＤ’［ｉ］＝ＬＤ［ｉ］×１００／ＩＤＭＡＸとして、ＹＭＣＫ階調変換テーブルへの出力値ＬＤ［ｉ］を置き換える。これにより、階調変換テーブルの選択に際して、ＩＤＭＡＸを考慮せずに済む（ＳＴＥＰ２４７〜２４９）。
【００６８】
次に、図３１に基づいて画像処理用プリンタγ補正回路１０９に設定する階調変換テーブルと画像形成用プリンタγ補正回路１１２に設定する階調変換テーブルの合成方法を説明する。
（ＳＴＥＰ２５０）では上記のようにして作成されたプロコン階調変換テーブル・データをＩＰＵへ転送する。
（ＳＴＥＰ２５１）では画像形成用プリンタγ補正回路１１２にスルーの階調変換テーブルを設定し、画像形成用プリンタγ補正回路１１２による補正を二重に行わないようにする。
【００６９】

（ＳＴＥＰ２５２）では画像処理用階調テーブルと画像形成用階調変換テーブルとの合成を行う。これについて以下に説明する。基準となる階調曲線Ａに対し、画像形成用階調変換テーブルの階調変換をＢとする。階調曲線Ａを階調変換Ｂにより、階調変換を行った階調曲線をＥ＝Ｂ（Ａ）と表記する。また、文字、写真、地図などのモード毎に湾曲度を変える階調変換をｍｏｄｅとする。これは、具体的には、プログラム言語Ｃの書式を用いて概略を表記すると、〈リスト５〉ｃｏｎｓｔｍｏｄｅ＿ｍａｘ＝１０；
ｔｙｐｅｄｅｆｉｎｔＴａｂｌｅ［２５６］；
ＴａｂｌｅＡ，Ｅ［ｍｏｄｅ＿ｍａｘ］，Ｂ；
＊／ｆｕｌｌ（）：全体の湾曲度を変える処理、＊／Ｔａｂｌｅｆｕｌｌ（ｉｎｔｍｏｄｅ）
｛／＊ｍｏｄｅは、画質モードを指定する。＊／ｉｎｔｉ；
ｆｏｒ（ｉ＝０；ｉ＜＝２５５；ｉ＋＋）
Ｅ［ｍｏｄｅ］［ｉ］＝Ｂ［Ａ［ｍｏｄｅ］［ｉ］］；
ｒｅｔｕｒｎＥ；
｝
ｍａｉｎ（）
｛／＊ｍｏｄｅは、画質モードを指定する。＊／ｉｎｔｍｏｄｅ；
ｆｏｒ（ｍｏｄｅ＝０；ｍｏｄｅ＜ｍｏｄｅ＿ｍａｘ；ｍｏｄｅ＋＋）
Ｅ［ｍｏｄｅ］＝ｆｕｌｌ（ｍｏｄｅ）；
｝
なお、上記の例では、ｍｏｄｅ＿ｍａｘ＝１０としていることから、画質モードを１０持つことになる。１０の画質モードは、一例としてそれぞれ、原稿の種類に応じて、単色用の文字モード、写真モード、印刷インク用の文字モード、写真モード、印画紙用、インクジェット用、蛍光ペン用、熱転写原稿用、地図用、複写原稿用に対応する。ＳＴＥＰ２５３では合成した１０本の階調変換テーブルを画像処理用の階調変換テーブルとしてそれぞれのエリアに対応させて設定する。前述した図４の例では、テーブル１〜４の画像処理用の階調変換テーブルのそれぞれと、画像形成用の階調変換テーブルとを合成し、合成した階調変換テーブルをそれぞれテーブル１〜４に設定する（図３２）。次に、画像形成用プリンタγ補正回路１１２に設定する階調変換テーブルと画像処理用プリンタγ補正回路１０９に設定する階調変換テーブルとを合成して作成した階調変換テーブルに対し、平滑化処理を行い、階調変換テーブルの階調飛びを無くす方法について述べる。合成した階調変換テーブルの一例をＥ２として、

ＩＮの３と４の間に階調飛び２→５が見られ、疑似輪郭となる可能性がある。
【００７０】
上の表では、ＩＮは、階調変換テーブルへの入力値、ＯＵＴは、階調変換テーブルの出力値を表す。
このＥ２に対し、一例として、１×５の大きさを持つデジタル・フィルタによる処理を行う。
フィルタとして、以下に示す係数（１１）を用いる：

この処理をプログラム言語Ｃを用いて表記すると
〈リスト６〉のように表すことができる。
【００７１】

ここで、関数ｆｉｌｔｅｒｉｎｇ（Ｔａｂｌｅ Ｏｕｔ，Ｔａｂｌｅ Ｉｎ）の関数引き数の中のＴａｂｌｅ Ｉｎは、デジタルフィルタ処理を行う前の補正階調曲線（すなわち、ＬＵＴ）、Ｔａｂｌｅ Ｏｕｔは、デジタルフィルタ処理後の補正階調曲線である。
これにより前述Ｅ２は、より階調飛びの小さい階調変換テーブルＥ３になる。
【００７２】

次に、画像処理部で１種類の階調変換テーブルのみを使用する場合には、画像処理部の階調変換テーブル（画像処理用プリンタγ補正回路１０９）と、画像形成部の階調変換テーブル（画像形成用プリンタγ補正回路１１２）とを合成して得られた階調変換テーブルを画像形成部のプリンタγ補正回路１１２に設定し、画像処理用プリンタγ補正回路にスルーを設定することも可能である（図３３）。また、コピア時に、画像処理手段内に含まれる階調変換テーブルと、プリンタ部の階調変換テーブルの２つの階調変換テーブルにより変換する場合には、階調変換テーブルの隣り合う入力信号に対する出力値の差が大きい場合がある。この差は、無変換（スルー）の階調変換テーブルの場合には、隣り合う入力値の差は１である（入力値０に対する出力値は０、入力値１に対する出力値は１、同様に、入力値２５５に対する出力値は２５５であるので、出力値の差は常に１である）。差が大きいとは、１より大きい場合である。
【００７３】
このような階調変換テーブルを２回通した場合、出力値の差は１回の変換に比べて大きくなる場合がある。このような場合は、階調が滑らかであるべき画像に対しても、濃度差がはっきりと目視できるような画像、いわゆる、疑似輪郭が現れるという不具合が生じる場合がある。従って、図３１の（ＳＴＥＰ２５１）において、画像形成用プリンタγ補正回路１１２に設定する階調変換テーブルは、スルーの階調変換テーブルでなくても、ほぼ同等の機能を有する次のような階調変換テーブルを使用しても良い。

【００７４】
【発明の効果】
本発明によれば、画像形成部と画像処理部のそれぞれに階調変換テーブルを設けることにより、色再現を向上させ画質の安定性を確保しながら画像形成手段と画像読み取り手段とをそれぞれ独立した装置として使用することもできる。また、画像読み取り手段と画像形成手段との階調変換テーブルを分けることにより、画像形成手段毎の機械差によるバラつきを画像形成手段の階調変換テーブルにより抑え、一方、画像読み取り手段・画像処理部の階調変換テーブルに画像読み取り手段および画質モード毎に最適なパラメータを設定しておくことにより、画像読み取り手段と画像形成手段との独立性を高め、かつ必要とする機能を保証することができる。また、文字、写真、地図などのモード毎に画像形成手段の階調テーブルを用意することで、より原稿に忠実な画像を得ることができる。また、印刷データを画像形成手段が有する画像信号の切り替え装置へ入力し、画像形成手段が有する階調変換テーブルにより変換し、画像形成手段の機械毎のバラつき補正し、使用する画像形成手段によらずに印刷をすることにより、画像形成手段単独の画像形成装置であっても、画像読み取り手段を有する画像形成装置としても使用可能となる。
【００７５】
また、光学検出センサーの検出結果と表面電位センサーの検出結果とをあわせて使用することにより、画像形成手段の階調変換テーブル（プロコンγ）の補正をより高精度に行い、これにより、濃度変動を補正し、常に良好な濃度を再現し原稿に忠実な画像を得ることができる。また、数ステップに分けた画像変換テーブルの内、１つのステップはプリンタ内にあり、画質モード毎にγを持っていてモード毎にγを切り替えることができると共に、濃度設定によってγの傾きを調節することができる。また、画像形成装置内にある画像信号切り替え手段後の外部用階調変換テーブルと画像処理手段内に含まれる内部用階調変換テーブルとを合成して、外部用階調変換テーブルと内部用階調変換テーブルのいずれか一方に設定し、他方を無変換テーブルに設定することにより、階調変換テーブルを２回通すことにより生じる疑似輪郭の発生を防ぐことができる。また、合成後の階調変換テーブルに階調飛びが見られる場合に、平滑化処理を行うことにより、階調変換テーブルの階調飛びによる疑似輪郭の発生を防ぐことができる。
【００７６】
また、画像処理手段内の複数の階調変換テーブルのそれぞれを外部用階調変換テーブルと合成し、合成した階調変換テーブルを画像処理装置内の複数の領域に対応した階調変換テーブルのそれぞれに設定することにより、原稿内の複数の領域のそれぞれに対応して異なった階調変換テーブルを使用することができ、疑似輪郭の発生を防ぐことができる。また、外部用階調変換テーブルと画像処理手段内に含まれる複数の内部用階調変換テーブルとを合成して求めた階調変換テーブルを、外部用階調変換テーブルに設定するか、あるいは複数の内部用階調変換テーブルに設定し、さらに、もう一方の外部用階調変換テーブルもしくは内部用階調変換テーブルには無変換テーブル（スルー）を設定することにより、２つの階調変換テーブルにより変換することによる疑似輪郭の設定を防ぐことができる。
【図面の簡単な説明】
【図１】図２に示した複写機における画像処理部のブロック構成図。
【図２】本発明による複写機の一実施形態の構成図。
【図３】図２に示した複写機における制御系の説明図。
【図４】図２に示した複写機における階調変換テーブルの切り替え動作の説明図。
【図５】図２に示した複写機におけるレーザー変調回路の構成図。
【図６】画像処理用プリンタγ補正回路で行われる階調変換テーブルの作成方法のフローチャートである。
【図７】図６のＳＴＥＰ２０１における全体の湾曲度を選択する動作の説明図。
【図８】図６のＳＴＥＰ２０１における全体の湾曲度を選択する動作の説明図。
【図９】図６のＳＴＥＰ２０１における全体の湾曲度を選択する動作の説明図。
【図１０】画像濃度（階調性）の自動階調補正（ＡＣＣ）の動作フローチャートである。
【図１１】図２に示した複写機における操作部を示す図。
【図１２】図１１に示した液晶画面の操作パターンの表示例を示す図。
【図１３】図１１に示した液晶画面の操作パターンの表示例を示す図。
【図１４】転写材上に形成された濃度階調パターンを示す図。
【図１５】図１１に示した液晶画面の操作パターンの表示例を示す図。
【図１６】図１１に示した液晶画面の操作パターンの表示例を示す図。
【図１７】画像形成用プリンタγ補正回路に設定する階調変換テーブルの作成方法における現像特性の検知方法のフローチャートである。
【図１８】図１７に示した検知方法において感光体上に濃度パターンを形成する説明図。
【図１９】画像信号の補正方法を示す４元チャートである。
【図２０】実際の現像特性と検知値から求めた現像特性のずれを補うための補正方法のフローチャートである。
【図２１】実際の現像特性と検知値から求めた現像特性のずれを補うための補正方法の説明図。
【図２２】階調変換テーブルの取得方法のフローチャートである。
【図２３】初期設定時と所定時間後の経時変化した階調性を見かけ上変化していない様に補正する補正方法のフローチャートである。
【図２４】補正テーブルの記憶容量を少なくすることのできる階調性の補正方法の説明図。
【図２５】補正テーブルの記憶容量を少なくすることのできる階調性の補正方法のフローチャートである。
【図２６】ＲＧＢ信号の補正データを示す図。
【図２７】図１１に示した液晶画面の操作パターンの表示例を示す図。
【図２８】階調変換テーブル選択動作の説明図。
【図２９】ＡＣＣ実行時の階調変換テーブルの作成手順のフローチャートである。
【図３０】補正階調曲線作成に関してＲＯＭに記憶されている階調変換テーブルの選択方法のフローチャートである。
【図３１】画像処理用プリンタγ補正回路に設定する階調変換テーブルと画像形成用プリンタγ補正回路に設定する階調変換テーブルの合成方法のフローチャートである。
【図３２】画像処理用プリンタγ補正回路に設定する階調変換テーブルと画像形成用プリンタγ補正回路に設定する階調変換テーブルの合成方法の説明図。
【図３３】画像処理部の階調変換テーブル（画像処理用プリンタγ補正回路）と、画像形成部の階調変換テーブル（画像形成用プリンタγ補正回路）とを合成して得られた階調変換テーブルを画像形成部のプリンタγ補正回路に設定し、画像処理用プリンタγ補正回路にスルーを設定する場合の説明図。
【符号の説明】
１…複写機本体、 ２…ドラム、
３…帯電チャージャー、 ４…レーザー光学系、
５…黒現像装置、 ６、７、８…カラー現像装置、
９…中間転写ベルト、 １０…バイアスローラ、
１１…クリーニング装置、 １２…除電部、
１３…転写バイアスローラ、 １４…ベルトクリーニング装置、
１５…搬送ベルト、 １６…定着装置、
１７…排紙トレイ、 １８…コンタクトガラス、
１９…露光ランプ、 ２１…反射ミラー、
２２…結像レンズ、 ２３…イメージセンサアレイ、
３０…メイン制御部（ＣＰＵ）、 ３１…ＲＯＭ、
３２…ＲＡＭ、 ３４…レーザ光学系制御部、
３５…電源回路、 ３６…光学センサー、
３７…トナー濃度センサー、 ３８…環境センサー、
３９…感光体表面電位センサー、 ４０…トナー補給回路、
４１…中間転写ベルト駆動部、 ４２…操作部、
４４…剤攪拌部材、 ４５…現像スリーブ、
１０１…シェーディング補正回路、 １０２…エリア処理回路、
１０３…スキャナーγ変換回路、 １０４…画像分離回路、
１０５…ＭＴＦフィルター、 １０６…色変換ＵＣＲ処理回路、
１０７…変倍回路、 １０８…画像加工回路、
１０９…画像処理用プリンタγ変換回路、 １１０…階調処理回路、
１１１、１２３…インターフェースＩ／Ｆ・セレクタ、
１１２…画像形成部用プリンタγ変換回路、
１１３…プリンタ、
１１７…システムコントローラー、 １１８…ＢＵＳ、
１２０…カラースキャナ、
１２１、１２２…パターン発生回路、
１５１…ルックアップテーブル、
１５２…パルス幅変調回路、 １５３…パワー変調回路、
１５４…レーザーダイオード、 １５５…フォトディデクタ、
１５６…液晶画面、[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus such as a digital copying machine, a printer, and a fax machine. In particular, the printer unit and the scanner unit are also used as independent devices while improving color reproduction and ensuring the stability of image quality. The present invention relates to an image forming apparatus that employs a gradation conversion method for image data.
[0002]
[Prior art]
In general, in a digital image forming apparatus such as an electrophotographic copying machine, the image density and gradation reproducibility for reproducing a constant good image always include changes in environmental conditions, deterioration of durability, change with time, and image mode. It depends on various factors and fluctuates mainly based on fluctuations in the characteristics of the photoreceptor and developer. In order to suppress this image reproduction variation, various digital image forming methods have been proposed.
On the other hand, an image forming apparatus such as a color copying machine can be divided into a scanner unit that reads an image and a printer unit that forms an image.
When image data is created by an external computer and the image data is output to transfer paper, only the printer unit described above is required, and the scanner unit is not required.
Further, in order to read a document and obtain a copy, it is necessary to read the document with the scanner unit and output an image to the printer unit, so both the scanner unit and the printer unit are necessary.
In other words, depending on the user's application, necessary components differ depending on whether only the printer unit is required or whether both the scanner unit and the printer unit are required.
[0003]
[Problems to be solved by the invention]
When only the printer unit of the image forming apparatus is used as the printer, the correspondence between the input image signal and the output image density is in an appropriate state and is constant regardless of the machine used. Is desirable. On the contrary, when the image density output with respect to the input image signal is different for each machine, there is a problem in that the output image density and hue are different depending on the machine used when outputting the image. Therefore, a gradation conversion table for converting the gradation characteristics of the image signal for correcting variations in output characteristics for each machine in the printer unit is required. When used as a copying machine, it is necessary to set image processing parameters including characteristics of a scanner and conversion characteristics of a gradation conversion table corresponding to a document to be copied.
However, in any of the above cases, in the conventional image forming apparatus, the printer unit and the scanner unit are integrated and have one gradation conversion table for always outputting an accurate image as a copier. There was no choice but to use the device.
As an image forming apparatus for performing conventional gradation conversion as described above, there is an image forming method and apparatus described in JP-A-7-261479.
In this conventional image forming apparatus, when the image forming apparatus is used for a long period of time, the density obtained by reading the pattern on the image carrier and the density of the actually printed image may not match. When a uniform density is output over the entire surface of the recording material in the image forming apparatus, if density jumps are observed due to charging unevenness due to contamination of the charger, the density may change depending on the location even with the same density output. In order to eliminate the disadvantage that an optimum image cannot be obtained if the gradation data is used and fed back to the image forming conditions, the image quality is stabilized by a plurality of types of calibration. .
[0004]
That is, in this prior art, in an image forming method using an image forming apparatus having a reading unit for reading a document and an image forming unit for forming an image on a medium, a system including both the reading unit and the image forming unit is described. The first calibration is performed, and the second calibration of the image forming unit alone is performed. The first and second calibrations are executed based on different reference images, and the second calibration is executed after the first calibration.
In another form of the prior art, in the image forming apparatus, a first reading unit that reads a document, an image forming unit that forms an image read by the first reading unit on a medium, Based on data obtained by reading a second reading means for reading an image formed on the image forming apparatus, a signal generating means for generating an image signal representing a predetermined reference image, and the first reading means. First control means for controlling the image forming conditions of the forming means, and second control means for controlling the image forming conditions of the image forming means based on data obtained by reading the reference image by the second reading means And have.
In another form of this prior art, in the image forming apparatus, a toner image is formed on the image carrier based on the first reading means for reading the document on the document table and the image information read by the image reading means. Forming means, second reading means for optically reading the reflection density of the formed toner image, means for transferring the toner image onto the image bearing member on the recording material, and image characteristics on the recording material Forming at least one image pattern for determining the image pattern, reading the image pattern by the first reading unit, and controlling the image forming conditions based on the read data; and on the image carrier In addition, at least one image pattern toner image for determining image characteristics is formed, the reflection density of the formed toner image is read by the second reading means, and based on the read data, It has a second control means for controlling the image forming conditions, a. In the control by the first control means, when a recording material having a plurality of sizes is provided as the recording material, the larger size is recorded. The image forming condition is at least one of a laser light quantity, a laser emission time, a one-time charger potential, and a developing bias. Then, the control by the second control unit is performed using the image forming conditions set under the first control conditions.
[0005]
In another form of the prior art, in the image forming apparatus, a reading unit that reads a document, an image forming unit that forms an image read by the reading unit on a medium, and an image signal representing a predetermined reference image In order to indicate to the operator the signal generating means for generating the image and the procedure for causing the reading means to read the reference image formed by the image forming means in accordance with the image signal generated from the signal generating means. Display means for performing predetermined guidance display is provided.
However, in the above prior art (Japanese Patent Laid-Open No. 7-261479 “Image Forming Method and Apparatus”), the gradation conversion table is described on the premise that it is provided in one place. The parts could not be used as independent devices.
Therefore, even a user who needs only the printer function cannot purchase only the printer unit, and conversely, even a user who needs only the scanner function cannot purchase only the scanner unit.
Further, an image forming apparatus used as a copying machine requires a scanner section even if it uses only a printer section that prints print data from a host computer. This increases the cost for a user who uses only the printer unit.
Conventionally, correction of image formation characteristics due to environmental fluctuations or the like has been performed by automatic gradation correction, but changes in image density due to fluctuations in development characteristics could not be corrected. For example, when the toner adhesion amount on the image carrier increases due to a change in the characteristics of the developer, the maximum image density is exceeded before the laser writing value reaches 255, and the gradation of the high density portion cannot be guaranteed. There was a case.
[0006]
In addition, in an image forming apparatus such as a color copying machine, color reproducibility that is faithful to an original is required. As one condition for obtaining this faithful color reproducibility, a tone correction table which is a look-up table for converting an YMCK component image signal so as to form an electrostatic latent image on a photosensitive member with an appropriate laser light amount. It is necessary that (γ correction table) is an appropriate value.
Deterioration with time of the photoconductive drum and developer causes deviation from an appropriate state due to change with time and deterioration of the image forming unit such as a transfer belt in the image forming apparatus. In order to set the above gradation correction table again to an appropriate value, the transfer paper on which the YMCK color gradation patterns or color patches are recorded is read by a scanner, and the gradation characteristics of the printer unit are determined from the read values. A gradation correction table to be corrected is created. This is called automatic gradation correction (ACC: Auto Color Calibration).
However, even if the above-described automatic gradation correction is performed and correction is performed so that an electrostatic latent image is formed on the photosensitive member with an appropriate laser light amount, if the image forming apparatus is used over a long period of time, the development characteristics change. For this reason, the density at which the pattern on the image carrier is read may not match the density of the actually printed out image.
Even if the gradation pattern or color patch output during ACC is written and output with a preset value, the density may vary due to changes in development characteristics.
[0007]
The present invention has been made to solve the conventional problems as described above, and its purpose is to improve color reproduction and improve image quality by providing a gradation conversion table in each of the image forming unit and the image processing unit. It is an object to provide an image forming apparatus capable of using the printer unit and the scanner unit as independent devices while ensuring the stability of the image forming apparatus.
Another object of the present invention is to divide the gradation conversion table into a scanner unit and a printer unit, thereby suppressing variations due to machine differences between printers by the gradation conversion table of the printer unit. By providing optimal parameters for each scanner and image quality mode in the gradation conversion table, we provide an image forming device that can increase the independence of the scanner unit and printer unit and guarantee the required functions. It is to be.
Another object of the present invention is to provide an image forming apparatus capable of obtaining an image more faithful to a manuscript by preparing a printer gradation table for each mode such as text, photo, and map.
Another object of the present invention is to use the detection result of the optical detection sensor and the detection result of the surface potential sensor in combination to correct the gradation conversion table (procon gamma) of the printer unit with higher accuracy, Accordingly, it is an object of the present invention to provide an image forming apparatus capable of correcting density fluctuations, always reproducing a good density, and obtaining an image faithful to a document.
Another object of the present invention is to provide an image forming apparatus that has a printer gradation table for each image quality mode, can switch γ for each mode, and can adjust the inclination of the printer gradation table by density setting. Is to provide.
[0008]
Another object of the present invention is to synthesize the value of the gradation conversion table of the image signal switching means in the image forming apparatus and the gradation conversion table included in the image processing means and set them to one of them. Accordingly, it is an object of the present invention to provide an image forming apparatus capable of preventing the occurrence of a pseudo contour caused by passing the gradation conversion table twice.
Another object of the present invention is to prevent the occurrence of pseudo contour due to gradation skipping in the gradation conversion table by performing smoothing processing when gradation skipping is found in the combined gradation conversion table. An image forming apparatus is provided.
Another object of the present invention is to synthesize each of the plurality of gradation conversion tables in the image processing unit with the gradation conversion table in the image forming unit, and to combine the combined gradation conversion table into a plurality of regions in the image processing apparatus. An image forming apparatus capable of preventing the occurrence of pseudo contours by using different gradation conversion tables corresponding to each of a plurality of areas in a document by setting each of the gradation conversion tables corresponding to It is to be. Another object of the present invention is to provide a gradation conversion table obtained by synthesizing a gradation conversion table in the image forming means and a plurality of gradation conversion tables included in the image processing means. Set in the tone conversion table, or set in a plurality of tone conversion tables included in the image processing means, and further, the tone in the other image forming means to which the tone conversion table obtained by synthesis is not set Setting a conversion table or setting a non-conversion table (through) in the gradation conversion table included in the image processing means prevents setting of a pseudo contour by conversion using two gradation conversion tables. It is an object of the present invention to provide an image forming apparatus capable of performing the above.
[0010]
[Means for Solving the Problems]
  In order to achieve the above object, according to the first aspect of the present invention, there is provided an image reading means for optically scanning and reading a document image placed at a reading position, an image signal from the image reading means, and an output image signal. Image processing means for outputting as image data writing means for writing information on the image carrier in response to the output image signal, switching the output image signal from the image processing means and the output image signal from the external device to switch the image An image forming apparatus comprising: an image switching unit that outputs to a writing unit; and an image forming unit that visualizes information on the image carrier with a plurality of colors of developer and forms an image on a transfer material. An internal gradation conversion table used for gradation conversion of the output image signal from the image processing means, and an image signal generation for generating a plurality of gradation patterns on the image carrier. Means for detecting the surface potential of the gradation pattern formed on the image carrier by the image signal generating means, and the output image signal from the external device is converted by the image switching means. Based on the optical detection means for detecting the amount of developer in the gradation pattern that has been toned and visualized on the image carrier, the detection result of the surface potential detection means, and the detection result of the optical detection means Created or selected by the image switching means,An output image signal from the image processing means subjected to gradation conversion using the internal gradation conversion table, orAnd an external gradation conversion table used for gradation conversion before an output image signal from the external device is output to the image writing means. According to the first aspect of the present invention, by using the detection result of the optical detection sensor and the detection result of the surface potential sensor in combination, the gradation conversion table (procon gamma) of the image forming means can be further corrected. This is performed with high accuracy, thereby correcting the density fluctuation, and always reproducing a good density and obtaining an image faithful to the original.
  According to a second aspect of the present invention, there is provided an image reading means for optically scanning and reading a document image placed at a reading position, and an image processing means for processing an image signal from the image reading means and outputting it as an output image signal. An image writing means for writing information on the image carrier in response to the output image signal, and an image to be output to the image writing means by switching between an output image signal from the image processing means and an output image signal from an external device. An image forming apparatus comprising: a switching unit; and an image forming unit that visualizes information on the image carrier with a developer of a plurality of colors and forms an image on a transfer material, onto the image carrier Image signal generating means for generating a plurality of gradation patterns, and gradation target data corresponding to a gradation pattern read signal formed on the image carrier by the image signal generating means are recorded. To be generated or selected on the basis of the storage means, the read signal of the gradation pattern read by the image reading means, and the gradation target data stored in the storage means, and the output image from the image processing means By the internal gradation conversion table used for gradation conversion of the signal and switching by the image switching means,An output image signal from the image processing means subjected to gradation conversion using the internal gradation conversion table, orAnd an external gradation conversion table used for gradation conversion before an output image signal from the external device is output to the image writing means.
[0011]
  Claims above2According to the image forming apparatus described in the above, one of the image conversion tables divided into several steps is in the printer, and the image forming unit has a gradation table for each image quality mode. The gradation table for the outside of the means can be switched, and the inclination of the gradation table of the image forming means can be adjusted by the density setting.
  Claim3The invention described in claim 1Or claim 2In the image forming apparatus described above, when only the image signal from the image processing unit is output by the image switching unit, the external gradation conversion table and the internal gradation conversion table are obtained by synthesis. The composite gradation conversion table is set to one of the external gradation conversion table and the internal gradation conversion table, and the other is set to a non-conversion table. Claims above3According to the image forming apparatus described above, the external gradation conversion table and the internal gradation conversion table in the image forming apparatus are combined and set to one of the gradation conversion tables, and the other is set. By setting the non-conversion table, it is possible to prevent the occurrence of a pseudo contour that occurs when the gradation conversion table is passed twice.
[0012]
  Claim4The described invention is claimed.3In the image forming apparatus described above, a smoothing process is performed on a combined gradation conversion table obtained by combining the external gradation conversion table and the internal gradation conversion table. Claims above4According to the image forming apparatus described in the above, when a gradation jump is seen in the combined gradation conversion table, a smoothing process is performed to prevent generation of a pseudo contour due to the gradation jump in the gradation conversion table. be able to.
  Claim5The described invention is claimed.3In the image forming apparatus according to claim 1, a combined gradation conversion table obtained by combining the external gradation conversion table and a plurality of gradation conversion tables included in the internal gradation conversion table is used as the internal gradation level. It is set in each gradation conversion table of the tone conversion table. Claims above5According to the image forming apparatus described in the above, each of the plurality of gradation conversion tables in the internal gradation conversion table is combined with the external gradation conversion table, and the combined gradation conversion table is combined with the plurality of gradation conversion tables in the image processing apparatus. By setting each of the gradation conversion tables corresponding to each area, it is possible to use different gradation conversion tables corresponding to each of a plurality of areas in the document, thereby preventing the occurrence of pseudo contours. it can.
[0013]
  Claim6The invention described in claim3In the image forming apparatus described in the above, a combined gradation conversion table obtained by combining the external gradation conversion table and the internal gradation conversion table is used as the external gradation conversion table or the internal gradation. One of the conversion tables is set, and the other conversion table is set for the other. Claims above6According to the image forming apparatus described in the above, the gradation conversion table obtained by combining the gradation conversion table for external use and the plurality of gradation conversion tables included in the image processing unit is used as the gradation conversion table for external use. Is set, or is set in a plurality of internal gradation conversion tables included in the image processing means, and the other external gradation conversion table or internal gradation conversion table is a non-conversion table (through). By setting, it is possible to prevent the occurrence of a pseudo contour due to conversion using two gradation conversion tables.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment in which the present invention is implemented in an electrophotographic copying machine (hereinafter simply referred to as a copying machine) which is an image forming apparatus will be described with reference to the drawings.
First, an outline of the mechanism of the copying machine main body 1 in one embodiment of the electrophotographic copying machine according to the present invention will be described with reference to the mechanism diagram of FIG.
In FIG. 2, there is a charge for charging the surface of the photoconductive drum 2 around an organic photoconductor (OPC) drum 2 having a diameter of 120 [mm] as an image carrier disposed almost at the center of the copying machine main body 1. Charger 3, laser optical system 4 that forms an electrostatic latent image by irradiating the surface of uniformly charged photosensitive drum 2 with semiconductor laser light, each color toner is supplied to the electrostatic latent image and developed, and each color A black developing device 5 that obtains a toner image every time, and three color developing devices 6, 7, and 8 of yellow Y, magenta M, and cyan C, and an intermediate that sequentially transfers toner images of each color formed on the photosensitive drum 2. The transfer belt 9, the bias roller 10 that applies a transfer voltage to the intermediate transfer belt 9, the cleaning device 11 that removes toner remaining on the surface of the photosensitive drum 2 after transfer, and the surface of the photosensitive drum 2 after transfer. Do Such as static eliminator 12 for removing load are sequentially arranged.
The intermediate transfer belt 9 has a transfer bias roller 13 for applying a voltage for transferring the transferred toner image to the transfer material, and a belt cleaning device 14 for cleaning the toner image remaining on the transfer material after transfer. Is arranged.
[0015]
A fixing device 16 that heats and pressurizes and fixes the toner image is disposed at the exit side end portion of the conveying belt 15 that conveys the transfer material peeled off from the intermediate transfer belt 9. A paper discharge tray 17 is attached to the outlet portion.
Above the laser optical system 4, a contact glass 18 serving as a document placement table disposed at the top of the copier body 1 and an exposure lamp 19 for irradiating scanning light to the document on the contact glass 18 are provided. The reflected light from the light is guided to the imaging lens 22 by the reflection mirror 21 and is incident on an image sensor array 23 of a CCD (Charge Coupled Device) which is a photoelectric conversion element. The image signal converted into an electrical signal by the CCD image sensor array 23 passes through an image processing section (see FIG. 1) and controls the laser oscillation of the semiconductor laser in the laser optical system 4.
[0016]
Next, a control system built in the copying machine will be described with reference to FIG.
As shown in FIG. 3, the control system includes a main control unit (CPU) 30, a predetermined ROM 31 and RAM 32 are attached to the main control unit 30, and the main control unit 30 has an interface. Via the I / O 33, the laser optical system control unit 34, the power supply circuit 35, the optical sensor 36, the toner density sensor 37, the environment sensor 38, the photoreceptor surface potential sensor 39, the toner replenishment circuit 40, the intermediate transfer belt drive unit 41, the operation The units 42 are connected to each other.
The laser optical system control unit 34 adjusts the laser output of the laser optical system 4, and the power supply circuit 35 gives a predetermined discharge voltage for charging to the charging charger 3, as well as the developing device 5, A developing bias having a predetermined voltage is applied to 6, 7 and 8, and a predetermined transfer voltage is applied to the bias roller 10 and the transfer bias roller 13.
[0017]
The optical sensor 36 includes a light emitting element such as a light emitting diode and a light receiving element such as a photo sensor which are disposed in the vicinity of the transferred area of the photosensitive drum 2, and a detection pattern latent image formed on the photosensitive drum 2. The toner adhesion amount in the toner image of the image and the toner adhesion amount in the background portion are detected for each color, and the so-called residual potential after the charge removal from the photosensitive member is detected.
The detection output signal from the photoelectric sensor 36 is applied to a photoelectric sensor control unit (not shown). The photoelectric sensor control unit obtains a ratio between the toner adhesion amount in the detection pattern toner image and the toner adhesion amount in the background portion, compares the ratio value with a reference value, detects a change in image density, and detects the toner density sensor 37. The control value is corrected.
Further, the toner density sensor 37 detects the toner density in the developing devices 5 to 8 based on the change in magnetic permeability of the developer present in the developing devices 5 to 8. The toner density sensor 37 compares the detected toner density value with a reference value, and when the toner density falls below a certain value and becomes a toner shortage state, a toner replenishment signal having a magnitude corresponding to the shortage is supplied to the toner. A function of applying to the supply circuit 40 is provided.
The potential sensor 39 detects the surface potential of the photosensitive member 2 that is an image carrier, and the intermediate transfer belt drive unit 41 controls the drive of the intermediate transfer belt.
The black developer 5 contains a developer containing black toner and a carrier, which is stirred by the rotation of the developer stirring member 44 and pumped up onto the sleeve by the developer stirring member 44. The supplied developer is magnetically carried on the developing sleeve 45 and rotates as a magnetic brush in the rotating direction of the developing sleeve 45.
[0018]
Next, the image processing unit of the copying machine will be described based on the block diagram of FIG. 1 (corresponding to claim 1).
In FIG. 1, 120 is a scanner, 101 is a shading correction circuit, 102 is an area processing circuit, 103 is a scanner γ conversion circuit, 104 is an image separation circuit, 105 is an MTF filter, 106 is a color conversion UCR processing circuit, and 107 is a scaling. Circuit 108, image processing (create) circuit, 109 image processing printer γ correction circuit, 110 gradation processing circuit, 111, 123 interface I / F / selector 112, image forming unit printer γ (hereinafter referred to as process controller) (referred to as γ) a correction circuit, 113 is a printer, and 121 and 122 are pattern generation circuits.
The scanner 120, the shading correction circuit 101, the area processing circuit 102, the interface I / F 123, the scanner γ conversion circuit 103, the image separation circuit 104, the MTF filter 105, the color conversion UCR processing circuit 106, the pattern generation circuit 121, and the scaling circuit. 107, an image processing (create) circuit 108, an image processing printer γ correction circuit 109, a gradation processing circuit 110, a pattern generation circuit 121, a CPU 30, a ROM 31, and a RAM 32 form a scanner / IPU unit (image reading and image processing unit). Then, a printer unit (image forming unit) is formed by the image forming unit printer γ (hereinafter referred to as a process controller γ) correction circuit 112, the printer 113, the I / F selector 111, the system controller 117, and the pattern generation circuit 122.
[0019]
An original to be copied is color-separated into R, G, and B by the color scanner 120 and read as a 10-bit signal as an example. The read image signal is corrected for unevenness in the main scanning direction by the shading correction circuit 101 and output as a 10-bit signal.
The area processing circuit 102 generates an area signal for distinguishing which area in the document the image data currently being processed belongs to. The parameters used in the subsequent image processing unit are switched according to the region signal generated by this circuit. These areas include color correction coefficients, spatial filters, and tone conversion that are optimal for each original, such as text, silver halide photographs (photographic paper), printed originals, inkjets, highlighters, maps, and thermal transfer originals. Image processing parameters such as a table can be set according to the image area.
The interface I / F 123 is used when an image read by the scanner is output to the outside. When used as a printer unit (image forming unit) and a scanner / IPU unit (image reading, image processing unit) like a copier, the image data read to the external device is taken out from the I / F selector 111 of the printer unit. You can also.
[0020]
In the scanner γ conversion circuit 103, a read signal from the scanner is converted from reflectance data to lightness data. In the image separation circuit 104, the character portion and the photograph portion are determined, and the chromatic / achromatic color is determined. The MTF filter 105 performs processing for changing the frequency characteristics of the image signal such as edge enhancement and smoothing according to the user's preference, such as a sharp image or a soft image.
The color conversion UCR processing circuit 106 corrects the difference between the color separation characteristics of the input system and the spectral characteristics of the output system color material, and calculates the amount of the color material YMC necessary for faithful color reproduction; It consists of a UCR processing unit for replacing the part where the three colors of YMC overlap with Bk (black). That is, the color correction process can be realized by performing a matrix operation as shown in the following equation.
Y a11 a12 a13 B
M = a21 a22 a23 G
C a31 a32 a33 R
Here, R, G, and B indicate the complements of R, G, and B. The matrix coefficient aij is determined by the spectral characteristics of the input system and the output system (color material). Here, the primary masking equation is taken as an example, but color correction can be performed with higher accuracy by using a quadratic term such as B2 and BG, or a higher-order term. The arithmetic expression may be changed depending on the hue, or the Neugebauer equation may be used. In any method, Y, M, and C can be obtained from the values of B, G, and R (or B, G, and R may be used).
[0021]
On the other hand, UCR processing can be performed by calculating using the following equation.
Y ′ = Y−α · min (Y, M, C)
M ′ = M−α · min (Y, M, C)
C ′ = C−α · min (Y, M, C)
Bk = α · min (Y, M, C)
In the above equation, α is a coefficient that determines the amount of UCR, and when α = 1, 100% UCR processing is performed. α may be a constant value. For example, when α is close to 1 in the high density portion and close to 0 in the highlight portion (low image density portion), the image in the highlight portion can be smoothed.
The scaling circuit 107 performs vertical and horizontal scaling, and the image processing (create) circuit 108 performs repeat processing and the like.
The image processing printer γ correction circuit 109 corrects the image signal in accordance with the image quality mode such as characters and photographs. Moreover, it is also possible to perform a background skip. The image processing printer γ correction circuit 109 has a plurality of (for example, 10) gradation conversion tables that can be switched in accordance with the area signal generated by the area processing circuit 102 described above. This gradation conversion table is an optimum gradation conversion table for each original such as characters, silver halide photographs (printing paper), printed originals, ink jets, fluorescent pens, maps, thermal transfer originals, etc. You can choose. (Corresponding to claim 2).
[0022]
  Next, dither processing is performed in the gradation processing circuit 110. Since the output of the gradation processing circuit 110 lowers the pixel frequency to ½, the image data bus has a 16-bit width (8-bit) so that data for two pixels can be simultaneously transferred to the printer unit. 2) of image data. The interface I / F selector 111 outputs image data read by the scanner 120 for processing by an external image processing device or the like, and outputs image data from an external host computer or image processing device by the printer 113. Has a switching function toTheAn image forming printer γ (procedure γ) correction circuit 112 converts an image signal from the interface 111 using a gradation conversion table, and outputs the converted signal to a laser modulation circuit described later. The printer includes the interface 111, the image forming printer γ correction circuit 112, the printer 113, and the controller 117, and can be used independently of the scanner / IPU. An image signal from the host computer 118 is input to the interface 111 through the printer controller 119, is subjected to gradation conversion by the image forming printer γ correction circuit 112, and image formation is performed by the printer 113, so that it can be used as a printer.
[0023]
The above image processing circuit is controlled by the CPU 30. The CPU 30 is connected to each part of the ROM 31 and RAM 32 and the scanner / IPU unit via the BUS 123. The CPU 30 is connected to the system controller 117 through a serial I / F, and commands from the operation unit 42 (see FIG. 3) and the like are transmitted through the system caller 117. Various parameters are set in each of the image processing circuits described above based on the transmitted image quality mode, density information, region information, and the like. The pattern generation circuits 121 and 122 generate gradation patterns used in the image processing unit and the image forming unit, respectively.
That is, as shown in FIG. 4, the designated area information on the document is compared with the reading position information at the time of image reading, and an area signal is generated from the area processing circuit 102. Based on the area signal, parameters used in the scanner γ conversion circuit 103, the MTF filter circuit 105, the color conversion UCR circuit 106, the image processing circuit 108, the image processing printer γ correction circuit 109, and the gradation processing circuit 110 are changed. FIG. 4 particularly shows the image processing printer γ correction circuit 109 and the gradation processing circuit 110 in detail.
[0024]
In the image processing printer γ correction circuit 109, the area signal from the area processing circuit 102 is decoded by the decoder 1, and a predetermined gradation table is selected from a plurality of gradation conversion tables such as characters and inkjets by the selector 1. select. In the example of the original in FIG. 4, an example in which a character area 0, a photographic paper area 1, and an inkjet area 2 exist is illustrated. For character area 0, character gradation conversion table 1, for photographic paper area 1, gradation conversion table 3 for photographic paper, and inkjet area 2 for inkjet The gradation conversion table 2 is selected (corresponding to claim 2).
The image signal subjected to gradation conversion by the image processing printer γ correction circuit 109 is used by the selector 2 based on the signal decoded by the decoder 2 again corresponding to the area signal in the gradation processing circuit 110. Switches gradation processing. As usable gradation processing, processing that does not use dither, processing that performs dithering, error diffusion processing, and the like are performed. The error diffusion process is performed on the ink jet document.
The image signal after the gradation processing is selected by the decoder 3 as line 1 or line 2 based on the reading position information. Line 1 and line 2 are switched every time one pixel differs in the sub-scanning direction. The data of line 1 is temporarily stored in a FIFO (First In First Out) memory located downstream of selector 3, and the data of line 1 and line 2 are output. Thereby, the pixel frequency can be reduced to ½ and input to the I / F selector 111.
[0025]
Next, laser modulation circuits prepared corresponding to the image data of line 1 and line 2 in FIG. 4 will be described with reference to FIG.
The writing frequency is 18.6 [MHz], and the scanning time for one pixel is 53.8 [nsec]. The 8-bit image data can be subjected to γ conversion by a look-up table (LUT) 151.
The pulse width modulation circuit (PWM) 152 converts the 8-bit image signal into an 8-value pulse width based on the 8-bit image signal, and the power modulation circuit (PM) 153 performs 32-value power modulation using the low-order 5 bits. The laser diode (LD) 154 emits light based on the modulated signal. The light intensity is monitored by a photo detector (PD) 155, and correction is performed for each bit. The maximum value of the intensity of the laser beam can be varied to 8 bits (256 levels) independently of the image signal.
The beam diameter in the main scanning direction with respect to the size of one pixel (this is 1 / e with respect to the maximum beam intensity at rest.² In 600 DPI, for one pixel 42.3 [μm], the beam diameter is 50 [μm] in the main scanning direction and 60 [μm] in the sub-scanning direction.
The laser modulation circuit described above is prepared for each of the image data of line 1 and line 2 in FIG. Here, the image data of line 1 and line 2 are synchronized and scan on the photosensitive member in parallel in the main scanning direction.
[0026]
Next, a method of creating a gradation conversion table (LUT) performed by the image processing printer γ correction circuit 109 will be described with reference to the flowchart of FIG.
In other words, the procedure for creating a tone conversion curve is as follows:
(STEP 201) The whole curvature is selected.
(STEP 202) The curvature h of the low image density (highlight) portion is selected.
(STEP 203) The curvature s of the high image density (shadow) portion is selected.
(STEP 204) An overall coefficient is used so that the image density becomes a desired value.
Multiply IDMAX.
Consists of.
[0027]
First, the processing of (STEP 201) will be described with reference to FIG.
For the reference gradation curve A, the gradation conversion that changes the overall curvature is B, and the gradation conversion that changes the curvature of the highlight area (low density area) is CH and the shadow area (high density area). Let CS be the gradation conversion that changes the degree of curvature.
A gradation curve obtained by performing gradation conversion on gradation curve A by gradation conversion B is represented by E, and this is represented as E = B (A).
Specifically, when the outline is expressed using the format of the programming language C,

It can be expressed as. Here, B is a function for changing the curvature of A.
[0028]
As an example of this function, in the case of an 8-bit image signal, a second-order Bezier function that satisfies 0 = B (0, n), 255 = B (255, n) (n is an arbitrary integer) is used. A Bezier function that satisfies the above conditions exists on the straight line L that intersects the straight line P0P1 and the straight line P0P1 connecting the start point P0 (0,0) and the end point P1 (255,255), and the straight line P0P1. It is expressed as a quadratic Bezier curve from the control point P2 with the distance d from the intersection with L as a parameter.
In the above function, the degree of curvature can be changed by making the distance d proportional to the integer curve which is an argument of the function B.
As an example, a case with respect to the straight line L1 orthogonal to the straight line P0P1 and an example with respect to the straight line L2 parallel to the vertical axis of the drawing will be described.
The control point in the first example is set such that the center point PC = (P0 + P1) / 2 = (127.5, 127.5) of the line segment P0P1 formed by the end points P0, P1 or (127, 127) or (128, 128), when the distance d to this point is a parameter, the control point P2 is
P2 (d) = PC + (− d / √2, d / √2) = (127.5−d / √2, 127.5 + d / √2) (FIG. 8). Thus, the gradation conversion curve P (d, t)
P (d, t) = P0 · t2 + 2 · P2 (d) · t · (1-t) + P1 · (1-t) 2 (1)
Given in.
[0029]
However, t is a parameter of 0 ≦ t ≦ 1. Since P (d, t) is given as a set (x, y) of an input x and an output y to the gradation conversion curve, from an integer A given as an argument to the function B (), x = A, T is obtained from Equation 1, and the obtained t is substituted into Equation (1) again to obtain the output value y.
Actually, instead of performing the calculation as described above every time, all the sets (0, x ≦ 255) of (x, y) are obtained in advance and stored in the ROM as a table. Calculation time can be omitted. This gradation correction table is held in the ROM 31 by changing the degree of curvature. The degree of curvature is given by the argument “curvature” to the function B () described above.
Thereby, <List 1> is rewritten as follows.
[0030]

In the above example, since table_max = 9, the number of tables having different curvatures is nine.
In the above example, a Bezier curve is used, but other high-order functions, exponential / logarithmic functions, and the like can be used as necessary.
[0031]
Next, the processing of (STEP 202) and (STEP 203) in FIG. 6 will be described.
In the same manner as described above, the curvature of the low image density (highlight) region and the high image density (shadow) region can be changed.
Rewriting <List 2> into a more general form:

Here, when conversion of the highlight conversion curve CH [h] and the shadow conversion curve CS [s] is performed,

Can also be expressed. Among them, curvature, h, and s are values that determine the curvature of the whole, highlight, and shadow portions, respectively.
[0032]
Note that the curves of the highlight portion and the shadow portion are created independently of each other.
A gradation conversion curve for changing the curvature of a specific density area, such as a highlight area and a shadow area, is generated as follows.
A straight line L intersecting the straight line P0P1 and P0P1 connecting the start point P0 and the end point P1, and a control point P2 that exists on the straight line L and has a distance d from the intersection point of the straight line P0P1 and the straight line L as a third order. A tone conversion curve is generated using a Bezier curve.
Here, as an example, a case with respect to a straight line L1 orthogonal to the straight line P0P1 and an example with respect to a straight line L2 parallel to the vertical axis of the drawing will be described.
As shown in FIG. 9, the conversion curve that changes the gradation characteristics of the highlight region is generated as follows as an example. The start point P0 and the end point P1 are P0 = (0, 0) and P1 = (255, 255), respectively, and the first control point P2 is P2 = (32, 32).
The control point P3 in the first example is P3 (d) = (16,16) + (− d / √2, d / √2) with the distance d from the intersection of the straight line P0P1 and the straight line L1 as a parameter. To do.
[0033]
The control point P3 in the second example is set to P3 (d) = (16, 16) + (0, d) with the distance d from the intersection of the straight line P0P1 and the straight line L2 as a parameter.
Using the above P0 to P3, the gradation conversion curve P (d, t) is
P (d, t) = P0 * t3 + 3 * P2 * t2 * (1-t) + 3 * P3 (d) * t (1-t) 2 + P1 * (1-t) 3 (2)
Given in.
Here, the end point is P1 = (255, 255), but the end point P1 is a point on the line segment m: (0, 0) − (255, 255), such as P1 = (64, 64). At this time, a line segment that is not included in the line segment P0P1 on the line segment m is used as an identity conversion as it is as a gradation conversion, and other areas are specified density areas such as a highlight area and a shadow area. It acts as a gradation conversion curve for changing the degree of curvature.
[0034]
Next, automatic gradation correction (ACC: Auto Color) of image density (gradation)
The operation of Calibration) will be described based on the flowchart of FIG.
When the ACC menu is called on the liquid crystal screen 156 of the operation unit (FIG. 11), the screen shown in FIG. 12 is displayed. When [Execute] of automatic gradation correction for copy use or printer use is selected, the screen shown in FIG. 13 is displayed. When the copy use is selected, the gradation correction table used when using the copy is changed. When the printer is used, the gradation correction table when the printer is used is changed based on the reference data.
Here, when the print start key is selected, a plurality of density gradation patterns corresponding to the respective image quality modes of YMCK, characters, and photographs are formed on the transfer material as shown in FIG. 14 (STEPs 205 and 206, 207). This density gradation pattern is stored and set in the IPU ROM 31 in advance. The pattern write values are 16 patterns of 00h, 11h, 22h, ..., EEh, FFh in hexadecimal notation. In the figure, patches for five gradations are displayed excluding the background portion, but any value can be selected from the 8-bit signals of 00h-FFh. In the character mode, a dithering process such as pattern processing is not performed, and a pattern is formed with 1 bit 256 gradations. In the photographic mode, the sum of the writing values of two pixels adjacent to each other in the main scanning direction is distributed and laser writing is performed. A value is formed. That is,
The pattern processing when the write value of the first pixel is n1, and the write value of the second pixel is n2,
When n1 + n2 ≦ 255,
First pixel write value: n1 + n2, second pixel write value: 0
If n1 + n2> 255,
Write value for the first pixel: 255, write value for the second pixel: n1 + n2-255, or
When n1 + n2 ≦ 128,
First pixel write value: n1 + n2, second pixel write value: 0
If 128 <n1 + n2 ≦ 256,
Write value of the first pixel: 128, write value of the second pixel: n1 + n2−128256 <n1 + n2 ≦ 383,
First pixel write value: n1 + n2-128, second pixel write value: 128383 <n1 + n2,
The first pixel write value: 255, the second pixel write value: n1 + n2-255, etc. are distributed. In addition to this, pattern processing actually used at the time of image formation is used.
[0035]
After the pattern is output on the transfer material, the screen shown in FIG. 15 is displayed on the operation screen so that the transfer material is placed on the document table 118.
When the transfer material on which the pattern is formed is placed on the document table (STEP 208 in FIG. 10) and the reading start is selected, the scanner runs and reads the RGB data of the YMCK density pattern (STEP 209). At this time, the data of the pattern portion and the data of the background portion of the transfer material are read. The read value of the pattern is corrected using RGB correction values described in detail later (STEPs 210 and 211).
When it is selected that the processing using the background data is performed (STEP 212), the background data processing is performed on the read data (STEP 213), and when the reference data is corrected (STEP 214), the high image density for the reference data is set. After performing the processing (STEP 215), the YMCK gradation correction table is created and selected (STEP 216).
The above processing is performed for each color of YMCK (STEP 217) and for each picture quality mode of photographs and characters (STEP 218). During processing, the screen shown in FIG. 16 is displayed on the operation image.
If the result of image formation using the YMCK tone correction table after the end of processing is not desirable, the [Undo] key is shown in FIG. 12 so that the YMCK tone correction table before processing can be selected. Is displayed in the screen shown in.
[0036]
Next, background correction will be described.
There are two purposes for the background correction process. One is to correct the whiteness of the transfer material used during ACC. This is because even if an image is formed on the same machine at the same time, the value read by the scanner differs depending on the whiteness of the transfer material used. Disadvantages without correction include, for example, when recycled paper or the like with low whiteness is used for this ACC, since recycled paper generally has a lot of yellow components, yellow gradation correction tables are created when a yellow tone correction table is created. Correct so that the component is reduced. In this state, when copying is next performed on art paper or the like having high whiteness, an image with a small amount of yellow components may be obtained and a desired color reproduction may not be obtained.
As another reason, when the thickness of the transfer paper (paper thickness) used at the time of ACC is thin, a color such as a pressure plate for pressing the transfer material is seen through and read by the scanner. For example, when an automatic document feeder called ADF (Auto Document Feeder) is installed instead of the pressure plate, a belt is used for conveying the original, but depending on the rubber material used, The whiteness is low and there is a slight gray taste. For this reason, the read image signal is also read as an image signal that is apparently high, so that when the YMCK tone correction table is created, the image signal is created so as to be thinner. In this state, when a transfer sheet having a thick paper thickness and poor transparency is used, an image having a low overall density is reproduced, so that a desirable image is not necessarily obtained.
[0037]
In order to prevent the above problems, the read image signal of the pattern portion is corrected from the read image signal of the paper background using the image signal of the paper background portion.
However, there is an advantage even when the above correction is not performed, and when using transfer paper that has a lot of yellow components, such as recycled paper, color correction is performed for colors that contain yellow components without correction. May improve. In addition, when only the transfer paper having a thin paper thickness is always used, there is an advantage that the gradation correction table is created in a state matched to the thin paper.
As described above, the correction of the background portion can be turned ON / OFF according to the user's situation and preference.
[0038]
  Next, a method of creating a gradation conversion table set in the image forming printer γ correction circuit 112 will be described.TheFirst, a development characteristic detection method will be described with reference to the flowchart of FIG. In STEP 219, np (in this case, np = 12) density gradation patterns are formed on the photoreceptor 2 as shown in FIG. The surface potential Vsi (i = 1, 2,..., Np) of the photosensitive member is read by the surface potential sensor 39 (STEP 220), and developed by the developing device to be visualized (STEP 221). Next, the detection output Vpi (i = 1, 2,..., Np) of the toner image on the photoconductor 2 is obtained by the optical sensor 36 present on the downstream side in the rotation direction of the photoconductor 2 (STEP 222). As an example, the laser output used for detection is 00 (H), 10 (H), 20 (H), 30 (H), 40 (H), 50 (H), as the value of the image signal (in hexadecimal notation), 60 (H), 70 (H), 90 (H), B0 (H), D0 (H), and FF (H) are used. The sum of the image signals of every two pixels in the main scanning direction is allocated to the two pixels according to the value as follows. That is, the image signal of the first pixel is N₁The image signal of the second pixel is N₂, The processed image signal of the first pixel is N₁'The image signal of the second pixel is N₂‘N’₁+ N₂≤ N when FF (H)₁'= N₁+ N₂N₂'= 0N₁+ N₂> N when FF (H)₁'= FF (H)
N₂'= N₁+ N₂-FF (H)
  Next, a method for correcting an image signal will be described with reference to FIG. In the graph a), the vertical axis represents the laser output (or image output signal), and the horizontal axis represents the output of the optical sensor 36. This graph is obtained by forming np density gradation pattern latent images on the photosensitive drum 2, developing the latent image, and detecting the amount of reflected light of the toner image by the optical sensor 36. In graph b), the vertical axis represents the laser output as in a), and the horizontal axis represents the surface potential of the photoreceptor. This represents the light attenuation characteristic of the photoconductor, and is obtained by measuring the surface potential when an np density gradation pattern latent image is formed on the photoconductor drum 2 by a potential sensor, as in a). Graph c) represents a gradation conversion table used in the image forming unit, in which the horizontal axis represents an image input signal (which is, for example, an amount proportional to the density of the original image), and the vertical axis represents the laser output or It represents an image signal (image output signal) obtained by converting an image input signal using a gradation conversion table. Here, the image input signal has a resolution of 8 bits (256 values), and the amount of light written by the laser similarly has a resolution of 8 (-10) bits between the minimum value and the maximum value of the laser.
[0039]
In the figure, a represents the relationship between the laser output used during detection and the image input signal.
In the graph d), the vertical axis represents the toner adhesion amount on the photosensitive member 2, and the horizontal axis represents the output of the optical sensor 36, which represents the output characteristic of the optical sensor. This characteristic varies depending on the type of sensor to be used, the mounting angle, the distance from the photosensitive member 2, and the like, which are known in advance and are almost constant.
In the graph e), the vertical axis represents the toner adhesion amount, and the horizontal axis represents the surface potential of the photoreceptor 2. This represents the relationship between the surface potential of the photoconductor 2 and the toner adhesion amount on the photoconductor 2 (that is, development characteristics).
In the figure, h represents the DC component of the development bias.
Graph f) represents the relationship of the toner adhesion amount on the photoreceptor 2 to the image input signal.
The output VPi of the optical sensor 36 is converted into the toner adhesion amount (M / A) i [mg / cm 2] (i = 1, 2,..., Np) on the photoreceptor 2 using the relationship of the graph d). This is calculated | required by the method described below as an example.
[0040]
The reflected light of the toner image formed on the photoreceptor 2 is detected by the photoelectric sensor 36 and sent to the main control unit 30 as a detection signal. Assuming that VSP and VSG are the photoelectric sensor output from the toner adhering portion of the reference pattern portion and the output of the background portion, respectively, the adhering amount m per unit area of the toner adhering to the reference pattern₁ [G / cm² ]
m₁ = -1n (VSP / VSG) / β
β = −6.0 × 103 [cm² / G]
From this relationship, the toner adhesion amount is converted. Here, β is a constant determined by the photoelectric sensor and the toner, and the above value is the value of the black toner. The same can be done for yellow, cyan and magenta. Here, calculation is performed, but conversion may be performed using a lookup table created in advance.
By the above method, the relationship between the photoreceptor surface potential VSi and the toner adhesion amount (M / A) i on the photoreceptor is obtained, and the development characteristic i in the graph e) is obtained.
[0041]
However, as shown in the graph d), the output of the optical sensor is constant in a toner adhesion amount region ((M / A) ≧ (M / A) C) higher than a certain toner adhesion amount (M / A) C. The value VPMIN is indicated. On the other hand, for the image input signal equal to or larger than the image input signal of n in c) in the figure, the surface potential of the photosensitive member 2 actually decreases and the toner adhesion amount changes as shown in b). Nevertheless, the toner adhesion amount (M / A) on the photoreceptor is always a constant amount (M / A) C. Therefore, in the graph e), even if the actual development characteristic is c, the development characteristic obtained from the detection result is i, and there is a deviation between the actual value c and the detected value i. Arise.
[0042]
Here, in order to compensate for the deviation between the actual development characteristic and the development characteristic obtained from the detected value, the following correction is performed (see FIG. 20).
When the detection value VPi of the optical sensor with respect to the image signal i is equal to or greater than the predetermined value VPc, the detected value is converted into the toner adhesion amount on the photosensitive member 2 or an amount (M / A) i substantially proportional to the amount (FIG. 20). STEP 223). From these values, a relational expression between the output value VSi of the surface potential sensor and (M / A) i is obtained. Here, using the linear equation,
(M / A) i = a × VSi + b (VPi ≧ VPc) (2)
Alternatively, the DC component of the development bias is VDC,
(M / A) i = a × (VSi−VDC) + b (VPi ≧ VPc)
Is used. Here, a and b are coefficients, which are determined from the values of VSi and (M / A) i using a method such as a least square method (STEPs 224 and 225). Here, if the toner adhesion amount on the photoreceptor 2 where the output value of the optical sensor 36 is VPc is (M / A) C, the adhesion amount range satisfying (M / A) i ≦ (M / A) C is established. Is the same.
[0043]
In a region where the toner adhesion amount on the photoconductor 2 is lower than a certain value (M / A) MIN, there may be a large deviation from the linear relationship between the toner adhesion amount and the surface potential on the photoconductor 2. In order to prevent this, the coefficients a and b in the above-described equation (2) are determined for the detection result of the toner adhesion amount on the photoreceptor satisfying (M / A) MIN ≦ (M / A) ≦ (M / A) C. (STEP 226).
Here, the toner adhesion amount is used, but the detection output of the optical sensor 36 corresponding to (M / A) MIN is defined as VPMAX.
VPc ≦ VP ≦ VPMAX
The coefficients a and b in the above-described equation (2) may be determined from the toner adhesion area corresponding to the toner adhesion area that satisfies the conditions (STEP 227 and 228).
The above flowchart is shown in FIG.
[0044]
In the above example, whether or not to use the value is determined from the magnitude relationship of the detection output of the optical sensor 36 with respect to the predetermined value VPc. This VPc is the toner adhesion amount (M / A) of the photosensitive member 2 as follows. The absolute value | ΔVP / Δ (M / A) | of the change amount ΔVP of the output value VP of the light detection means with respect to the change Δ (M / A) of the light is a predetermined value | ΔVP / Δ (M / A) ｜₀ Is equal to (M / A) C, and the output value of the optical sensor at that time is VPc, the above-described method can be used. in this case,
| ΔVP / Δ (M / A) | ≧ | ΔVP / Δ (M / A) |₀ The toner adhesion amount area on the photoreceptor satisfying
Corresponds to (M / A) ≦ (M / A) C, and | ΔVP / Δ (M / A) |> | ΔVP / Δ (M / A) |₀ But,
This corresponds to (M / A) i ≦ (M / A) C (FIG. 21).
The detection result c of the graph f) is obtained from the development characteristic i and the image signal thus obtained.
A detection characteristic c of the fourth quadrant was obtained for the gradation conversion table of the first quadrant a. Using this result, a corrected gradation conversion table b) for obtaining the ideal characteristic d can be obtained.
The gradation conversion table obtained as described above is used as an image forming gradation conversion table.
[0045]
The above flowchart is shown in FIG.
In FIG. 22, (STEP 229) A reference pattern is created on the photoreceptor.
(STEP 230) The formed reference pattern is detected by a photoelectric sensor and a surface potential sensor.
(STEP 231) The development characteristics are predicted from the detected result and the output value (image signal) of the laser used at the time of pattern formation.
(STEP 232) A gradation conversion table for the image forming unit is created from the predicted development characteristics.
Next, a correction method when the development characteristics change with time after a predetermined time from the initial setting will be described.
The relationship between the image input signal at the time of setting and the laser output is a, the detection data of the optical sensor for the laser output at that time is b, and the toner adhesion on the photoconductor 2 to the surface potential of the photoconductor 2 obtained by the method described above. The relationship between the amounts is c, and the relationship between the image input image signal and the toner adhesion amount on the photoreceptor 2 is d.
If the detection output of the optical sensor 36 after a predetermined time becomes b ′, this indicates that the development characteristic has changed from c to c ′, and as a result, the toner adhesion on the photoconductor 2 to the image input image signal. The relationship of quantity represents that d ′. Therefore, the gradation has changed between the initial stage and a predetermined time.
It is not desirable from the viewpoint of image reproduction that the gradation changes after the initial time and a predetermined time. Therefore, this is corrected as follows.
It is assumed that the relationship between the image input signal n of the first quadrant and the laser output P is a linear relationship as shown by a at the time of initial setting. That is, the amount of laser light at the time of the image signal FFH is PMAX,
P = PMAX / FF (H) × n
It is.
[0046]
When the relationship between the image input signal and the laser output is a, the laser output with respect to the image input signal i is Pi, and the toner adhesion amount on the photoreceptor 2 is a predetermined time from the initial (M / A) i (0). Later, it has changed to (M / A) i (t). On the other hand, with respect to the image input signal j, the laser output is Pj, and the toner adhesion amount on the photoreceptor 2 with respect to this laser output is (M / A) after a predetermined time from the initial (M / A) j (0). It changes to j (t).
Among the above cases, particularly when (M / A) i (0) = (M / A) j (t), the relationship between the image input signal i and the laser output after a predetermined time is changed from i → Pi. By changing from i → Pj, the image density with respect to the image input signal apparently does not change after the predetermined time from the initial time.
In this way, the characteristic a ′ can be obtained by performing similar processing using np pieces of detection data. At this time, the value between the actual measurement points can be given by performing linear interpolation or interpolation by a spline curve or the like.
Alternatively, the above-mentioned calculation is performed for the detected np data points or some of them without performing the calculation for all the points, and the lookup table stored in the ROM 31 using the calculated values. May be selected and used as the correction characteristic a ′.
Thereby, it is possible to correct so that the gradation which has changed with time after the initial setting and after a predetermined time does not change apparently.
[0047]
The above flowchart is shown in FIG.
That is, the development characteristic is detected in STEP 233 of FIG. 23, the detection output is stored as (M / A) i (t) in STEP 234, and the development characteristic is corrected in STEP 235.
When the laser output corresponding to the image input signal FFH is PMAX (t) and the laser output of the image input signal FFH at the initial setting is PMAX (0) in the characteristic a ′ after a predetermined time obtained by the above method. As a correction method when PMAX (0) ≠ PMAX (t), PMAX (t) = Pk (0) while maintaining the resolution between the laser output P00 and PMAX (0) in the image signal 00H. There are two correction methods, that is, a case where the image input signal k up to) is used and a case where a resolution of 8 to 10 bits is provided between PMAX (t). In the present invention, either of the above two methods may be used. However, in the former case, the maximum value of the laser light amount is not changed, so that the control of the image forming condition is simplified. There is a disadvantage that it decreases.
[0048]
In addition to the above correction method, as shown in FIG. 24, by changing the development bias, which is one of the image forming conditions, from e → e ′, the development start potential (here, the adhesion amount (M / A)) The surface potential at MIN) can be changed from f ′ to f ″. As a result, the relationship between the surface potential on the photosensitive member 2 and the toner adhesion amount on the photosensitive member changes from C ′ to C ″. Here, e 'is selected to coincide with the initial development start potential e.
In this case, the correction amount (the difference between a and a ′ and the difference between a and a ″) between the image input signal and the laser output is a ′ → a ″ (FIG. 19) with respect to a → a ′. FIG. 24) is small. In the case of the method of selecting a correction table, since the correction width can be reduced by using this method, there is an advantage that the storage capacity of the correction table can be reduced.
In addition to the developing bias, the same effect can be obtained by controlling the charging potential of the photosensitive member 2 and the like. A flowchart in this case is shown in FIG.
That is, the development characteristics are detected in STEP 236 of FIG. 25, the detection output is stored as (M / A) i (t) in STEP 237, the image forming conditions are changed in STEP 238, and the development characteristics are corrected in STEP 239.
[0049]
  Next, a method for creating a gradation correction table set in the image processing printer γ correction circuit 109 will be described.TheLD [i] (i = 0, 1,..., 9) is the gradation pattern write value generated by the pattern generation circuit 121, and the read value (r [t] [i], g [t] [i], b [t] [i]) (t = Y, M, C, ork, i = 0, 1,..., 9). When forming a gradation pattern, the gradation conversion table obtained by the above-described method is set in the gradation conversion table for the image forming unit. Instead of (r, g, b), it is expressed by lightness, saturation, hue angle (L *, c *, h *), lightness, redness, blueness (L *, a *, b *), etc. Also good. The reference data includes an input value n (n = 0, 1, 2,..., 255) to the gradation conversion table and a reading value (r [t] [i], g [t] [i], b [ t] [i]).
[0050]
Represent the reference data as follows:
Formula 3
Ar [t] [n] (0 ≦ n ≦ 255, t = Y, M, C, or k)
Ag [t] [n] (0 ≦ n ≦ 255, t = Y, M, C, or k)
Ab [t] [n] (0 ≦ n ≦ 255, t = Y, M, C, or k)
Here, Ar, Ag, and Ab are reference data for the Red signal, the Green signal, and the Blue signal, respectively, and YMCK represents the color of the toner.
The above expression 3 represents that the reference data corresponding to 256 values from 0 to 255 values is held in the memory by 8-bit signal processing, which can be taken by the input value to the gradation conversion table. ing.
By storing 256 reference data in the memory as described above, the processing described later can be simplified, but in order to save the amount of memory for storing the reference data, n [0] = 0, n [i] = 26 × I−5 (I = 1, 2,..., 10) as an example, a number of n [i] (16 in this case) Pair with corresponding reference data (Equation 3)
Formula 4
n [i] (0 ≦ n [i] ≦ 255, i = 0, 1, 2,..., 10)
Ar [t] [n [i]] (0 ≦ n [i] ≦ 255, i = 0, 1, 2,..., 10, t = Y, M, C, or k)
Ag [t] [n [i]] (0 ≦ n [i] ≦ 255, i = 0, 1, 2,..., 10, t = Y, M, C, or k)
Ab [t] [n [i]] (0 ≦ n [i] ≦ 255, i = 0, 1, 2,..., 10, t = Y, M, C, or k)
Are stored in the memory, and reference data Ar [t] [n for n (n = 1 to 20 in the above example) other than n [i] (I = 0, 1, 2,..., 10) is stored. [I]] may be calculated by performing interpolation, as will be described later. As an example, n [i], n [i + 1] satisfying n [i] ≦ n ≦ n [i + 1] (for n = 1 to 20, I = 0, n [0] = 0, n [1 ] = 21) is obtained by performing interpolation using reference data Ar, g, b [t] [n [i]], Ar, g, b [t] [n [i + 1]].
[0051]
On the other hand, in the RAM 32, the ratio of the RGB component size in the reference data of the pattern reading value for each of the YMCK toners,
K [s] [t] {s = R, G, or B; t = Y, M, C, or K}
Is remembered. K [s] [t] takes a value near 1. However, it is held as integer data in the copying machine as follows.
K [s] [t] = K1 [s] [t] / 2ⁿ (A K1 [s] [t] is an integer) For example, n = 10, 2 ^ n (meaning 2 to the power of n) = 1024, and the like.
[0052]
FIG. 26 shows an example of K [s] [t] values that are correction values for RGB signals.
The correction data of the RGB signals shown in FIG. 26 are displayed on the display screen 156 of the operation unit of the copying machine main body 1 as shown in FIG. 27, and these numerical values are obtained by pressing the corresponding part of the display location with a finger. Can be entered. The input data is stored in the RAM 32.
As an example, a case where t = C (cyan) will be described. RGB component of cyan toner reference data
Formula 5
Ar1 [C] [n [i]] = Ar [W] + (Ar [C] [n [i]] − Ar [W] × k [r] [C]
Ag1 [C] [n [i]] = Ag [W] + (Ar [C] [n [i]] − Ar [W] × k [g] [C]
Ab1 [C] [n [i]] = Ab [W] + (Ar [C] [n [i]] − Ar [W] × k [b] [C]
(I = 0, 1, 2, ..., 10)
And correct. Here, (Ar1 [C] [n [I]], Ag1 [C] [n [i]], Ab1 [C] [n [I]]) are respectively the RGB components of the corrected reference data. (Ar [t] [n [I]], Ag [t] [n [i]], Ab [t] [n [i]]) is reference data before correction.
[0053]
Ar [W], Ag [W], and Ab [W] are RGB signals when white (the brightest color for the scanner to be used) is read. This value is in the range of 0 to 255 when the read value is an 8-bit signal, where 0 is the darkest image density (scanner CCD when reading an object with low reflectance or transmittance). The 255 value has the brightest image density (the amount of light detected by the CCD of the value scanner when reading an object having a high reflectance or transmittance), and a value in the vicinity of the 255 value.
Although the accuracy is slightly lower than the above case, in actual use,
Ar [W] = Ar [C] [0], Ag [W] = Ag [C] [0], Ab [0] = Ab [C] [0]
It is good.
Here, Ar [C] [0], Ag [C] [0], and Ab [C] [0] are values obtained by reading the background portion of the paper.
Here, when reading the background portion of the paper, several sheets of paper are stacked on the back side of the paper (so-called white back), so that the back of the paper is not darkened to prevent the accuracy from being lowered. be able to.
[0054]
As another example, similarly, an example in the case of t = C (cyan) is shown.
Equation 6

As a matter of fact, it can be processed in practice.
However, when I = 0, n [0] = 0, that is, when the input value to the gradation correction table is 0, the correction of Expression 6 is not performed.
The values of k [r] [C], k [g] [C], k [b] [C] used in Equation 6 and k [r] [C], k [g] [C] used in Equation 5 ], K [b] and [C] are not the same numerical value, and it is necessary to change the numerical value to an appropriate value depending on the expression used.
In order to simplify the processing, the above formula (Ar1 [C] [n [I]], Ag1 [C] [n [i]], Ab1 [C] [n [i]]) is newly added. (Ar [t] [n [i]], Ag [t] [n [i]], Ab [t] [n [i]]) are used below.
[0055]
Next, a method of generating a gradation conversion table (LUT) that is created and selected during ACC execution and set in the image processing printer γ correction circuit 109 will be described.
Since the complementary color image signals of the YMC toners are blue, green, and red, respectively, the reference data Ar [t] [i], Ag [t] [i], Ab [ Among the t] [i], the complementary color reference data Ab [t] [i], Ag [t] [i], Ar [t] [i] for each toner are used.
The above handling is effective when the spectral (reflectance) characteristics of the toner to be used do not change significantly (that is, when the color does not change).
To simplify the following description, A [t] [n [i]] (0 ≦ n [i] ≦ 255; i = 1, 2,..., 10; t = C, M, Y) is used. Represent. For black toner, sufficient accuracy can be obtained by using any of RGB image signals, but here, a G (green) component is used.
Similarly, the read signal is represented by a [t] [i] (i = 0, 1,..., 9; t = C, M, Y, K) using only the complementary color image signal.
[0056]
Also, reference data A [t] [i] and LD write value a [t] [i] for a certain color toner t (t = C, M, Y, K) are expressed as A [i ] And a [i].
The YMCK gradation conversion table is obtained by comparing a [LD] and A [n] described above. Here, n is an input value to the YMCK gradation conversion table, and the reference data A [n] obtained by correcting the RGB signal is output as a laser writing value LD [i] after YMCK gradation conversion of the input value n. This is the target value of the read image signal obtained by reading the YMC toner pattern read by the scanner. Here, there are two types of reference data in which RGB signals are corrected: reference data that is corrected according to image density that can be output by the printer, and reference data that is not corrected. The determination as to whether or not to perform correction is made based on data for determination described later stored in the ROM 31 or RAM 32 in advance. This correction will be described later.
The laser output LD [n] corresponding to the input value n to the YMCK gradation conversion table is obtained by obtaining the LD corresponding to the reference data A [n] described above.
[0057]
By obtaining this with respect to the input values i = 0, 1,..., 255 (in the case of an 8-bit signal), a gradation conversion table can be obtained.
At this time, instead of performing the above processing on all values for the input values n = 00h, 01h..., FFh (hexadecimal number) for the YMCK gradation conversion table, n [i] = 0, 11h, 22h, ..., the above processing is performed for discrete values such as FFh, and for other points, interpolation is performed using a spline function or the like, or the YMCK gradation conversion table stored in the ROM 31 in advance is included. The closest pass through the set of (0, LD [0]), (11h, LD [11h]), (22h, LD [22h]), ..., (FFh, LD [FFh]) obtained by the above processing Select a table.
[0058]
The above processing will be described with reference to FIG. 28. The horizontal axis of the first quadrant (a) in the figure is the input value n to the YMCK gradation conversion table, and the vertical axis is the reading value of the scanner (after processing). Represents the reference data A [i] described above. The reading value of the scanner (after processing) is the RGB γ conversion (no conversion is performed here) for the value read by the scanner, and the average processing and addition processing of the read data at several places in the gradation pattern This is a later value, and is processed here as a 12-bit data signal in order to improve calculation accuracy.
The horizontal axis of the second quadrant (b) in the figure represents the reading value (after processing) of the scanner, like the vertical axis.
[0059]
The vertical axis of the third quadrant (c) represents the writing value of the laser beam (LD). This data a [LD] represents the characteristics of the printer unit. Further, the LD writing values of the pattern to be actually formed are 16 points of 00h (background), 11h, 22h,... EEh, FFh, and indicate skipping values. And treated as a continuous graph.
The graph (d) of the fourth quadrant is a YMCK gradation conversion table LD [i], and the purpose is to obtain this table.
The vertical axis and horizontal axis of the graph (f) are the same as the vertical axis and horizontal axis of the graph (d). When forming a gradation pattern for detection, the YMCK gradation conversion table (g) shown in the graph (f) is used.
The horizontal axis of the graph (e) is the same as that of the third quadrant (c), and represents the relationship between the LD writing value at the time of gradation pattern creation and the reading value (after processing) of the gradation pattern scanner. Represents a linear transformation for convenience.
Reference data A [n] is obtained for a certain input value n, and an LD output LD [n] for obtaining A [n] is used as an arrow in the figure using a gradation pattern read value a [LD]. Seek along.
[0060]
A calculation procedure will be described with reference to FIG. FIG. 29 is a flowchart showing a procedure for creating a gradation conversion table when ACC is executed.
(STEP 240) An input value necessary for obtaining the YMCK gradation conversion table is determined.
Here, n [i] = 11 [h] × i (i = 0, 1,..., Imax = 15).
(STEP 241) The reference data is corrected using the correction value of the RGB signal in the above-described procedure.
(STEP 242) The reference data A [n] is corrected according to the image density that can be output by the printer.
Here, the writing value of the laser that can obtain the maximum image density that can be created by the printer is FFh (hexadecimal number display), and the reading value m [FFh] of the pattern at this time is mmax. Reference data A [i] (i = 0, 1,..., I1) that is not corrected from the low image density side to the intermediate image density side, and reference data A [i] (i = , imax−1) (i2 ≧ i1, i2 ≦ imax−1), and reference data A [i] (i = i1 + 1,..., i2) to be corrected.
[0061]
In the following, a specific calculation method will be described on the assumption that the image signal is proportional to the document reflectance without RGB-γ conversion. Among the reference data that is not corrected, the difference between the reference data A [i2 + 1] having the lowest image density in the high image density portion and the reference data A [i1] having the highest image density in the low image density portion. Δref is obtained. That is,
Δref = A [i1] −A [i2 + 1] (7)
Here, Δref> 0 in the case of the reflectance linearity or lightness linear which does not perform the RGBγ conversion which is the inversion processing.
On the other hand, the difference Δdet is similarly obtained from the read value mmax of the pattern that can obtain the maximum image density that can be created by the printer unit. That is,
Δdet = A [i1] −mmax (8)
And
[0062]
This
From (7) and (8), the reference data A [i] (i = i1 + 1,..., I2) obtained by correcting the high density portion is

And
(STEP 243) The read image signal m [i] of the scanner corresponding to n [i] obtained in (STEP 240) is obtained from the reference data A [n].
Actually, the reference data A [n [j]] (0 ≦ n [j] ≦ 255, j = 0, 1,... Jmax, n [j] ≦ n [k] corresponding to the discrete n [j] for j ≦ k) is as follows. That is, j (0 ≦ j ≦ jmax) that satisfies n [j] ≦ n [i] <n [j + 1] is obtained.
In the case of an 8-bit image signal, calculation is performed when reference data is obtained as n [0] = 0, n [jmax] = 255, n [jmax + 1] = n [jmax] +1, and A [jmax + 1] = A [jmax]. It will be easy.
[0063]
M [i] is obtained from the following equation from j obtained as described above.
m [i] = A [j] + (A [j + 1] -A [i]). (n [i] -n [j]) / (n [j + 1] -n [j]) (10)
In addition, as for the interval of the reference data, the accuracy of the finally obtained gradation conversion table is higher when n [j] is as small as possible.
Here, interpolation is performed using a linear expression, but interpolation may be performed using a higher-order function or a spline function. In that case,
m [i] = f (n [i])
And In the case of k-order function,
[0064]
[Expression 1]

And so on.
(STEP 244) The LD write value LD [i] for obtaining m [i] obtained in (STEP 242) is obtained by the same procedure as in (STEP 242) (j-> k).
When image signal data that has not undergone RGBγ conversion is processed, a [LD] decreases as the value of LD increases. That is,
For LD [k] <LD [k + 1], a [LD [k]] ≧ a [LD [k + 1]]
It becomes.
[0065]
Here, the values at the time of pattern formation are 10 values of LD [k] = 00h, 11h, 22h,..., 66h, 88h, AAh, FFh, (k = 0, 1,..., 9). This is because the change in the reading value of the scanner with respect to the toner adhesion amount is large at an image density with a small amount of toner adhesion. Therefore, the interval between the pattern writing values LD [k] is narrowed. Since the change of the reading value of the scanner with respect to the amount of adhesion is small, reading is performed with a wide interval.
Advantages of this include that toner consumption can be suppressed compared to the case where the number of patterns is increased to LD [k] = 00h, 11h, 22h,..., EEh, FFh (16 points in total), etc. In the density region, the change in the LD writing value is small, and the reading value is likely to be reversed due to the influence of potential unevenness on the photoreceptor, toner adhesion unevenness, fixing unevenness, potential unevenness, etc. The pattern was formed with the LD write value as described above, because it is not necessarily effective in improving accuracy even if the width is narrowed.
[0066]
For LD [k] where a [LD [k]] ≧ m [i]> a [LD [k + 1]],
LD [i] = LD [k] + (LD [k + 1] −LD [k]) · (m [i] −a [LD [k]]) / (a [LD [k + 1]] − a [LD [ k]])
And
When 0 ≦ k ≦ kmax (kmax> 0) and a [LD [kmax]]> m [i] (when the image density of the target value obtained from the reference data is high),
LD [i] = LD [k] + (LD [kmax] −LD [kmax−1]) · (m [i] −a [LD [kmax−1]]) / (a [LD [kmax]] − a [LD [kmax-1]])
As a prediction, extrapolation is performed using a linear expression.
In addition to the linear expression, extrapolation may be performed by other methods such as logarithm.
Thereby, a set (n [i], LD [i]) (i = 0, 1,..., 15) of the input value n [i] and the output value LD [i] to the YMCK gradation conversion table is obtained. .
In (STEP 245), based on the obtained (n [i], LD [i]) (i = 0, 1,..., 15), interpolation is performed using a spline function or the like, or it is stored in the ROM 31. Select the gray level conversion table.
[0067]
Hereinafter, a method for selecting a gradation conversion table stored in the ROM 31 will be described with reference to FIG.
In (STEP 246), a coefficient IDMAX [%] to be applied to the entire gradation conversion table is obtained.
In the case of n [imax] = FFh, IDMAX = LD [imax] / FFh × 100 [%].
Here, as LD ′ [i] = LD [i] × 100 / IDMAX, the output value LD [i] to the YMCK gradation conversion table is replaced. Thereby, when selecting the gradation conversion table, it is not necessary to consider IDMAX (STEPs 247 to 249).
[0068]
  Next, a method for synthesizing the gradation conversion table set in the image processing printer γ correction circuit 109 and the gradation conversion table set in the image forming printer γ correction circuit 112 will be described with reference to FIG.The
  In (STEP 250), the process control gradation conversion table data created as described above is transferred to the IPU.
In (STEP 251), a through gradation conversion table is set in the image forming printer γ correction circuit 112 so that the correction by the image forming printer γ correction circuit 112 is not performed twice.
[0069]

In (STEP 252), the image processing gradation table and the image forming gradation conversion table are combined. This will be described below. The gradation conversion of the gradation conversion table for image formation is set to B with respect to the reference gradation curve A. A gradation curve obtained by performing gradation conversion on the gradation curve A by gradation conversion B is expressed as E = B (A). Further, the tone conversion for changing the degree of curvature for each mode, such as text, photo, and map, is defined as mode. Specifically, when the outline is expressed using the format of the programming language C, <List 5> constmode_max = 10;
typedefintTable [256];
Table A, E [mode_max], B;
* / Full (): processing to change the overall curvature, * / Tablefull (intmode)
{/ * Mode specifies the image quality mode. * / Inti;
for (i = 0; i <= 255; i ++)
E [mode] [i] = B [A [mode] [i]];
returnE;
}
main ()
{/ * Mode specifies the image quality mode. * / Intmode;
for (mode = 0; mode <mode_max; mode ++)
E [mode] = full (mode);
}
  In the above example, since mode_max = 10, ten image quality modes are provided. The ten image quality modes are, for example, according to the type of the original, respectively, a monochrome color mode, a photographic mode, a printing ink character mode, a photographic mode, a photographic paper, an inkjet, a fluorescent pen, and a thermal transfer original Corresponding to map and copy manuscript. In STEP 253, the combined 10 gradation conversion tables are set in correspondence with each area as a gradation conversion table for image processing. In the example of FIG. 4 described above, each of the gradation conversion tables for image processing in Tables 1 to 4 and the gradation conversion table for image formation are combined, and the combined gradation conversion tables are Tables 1 to 4, respectively. (FIG. 32). Next, the tone conversion table created by combining the tone conversion table set in the image forming printer γ correction circuit 112 and the tone conversion table set in the image processing printer γ correction circuit 109 is smoothed. Describes how to perform processing and eliminate gradation skip in the gradation conversion tableTheAn example of the combined gradation conversion table is E2,

A gradation skip 2 → 5 is seen between IN 3 and 4, and there is a possibility of a pseudo contour.
[0070]
In the above table, IN represents an input value to the gradation conversion table, and OUT represents an output value of the gradation conversion table.
For this E2, as an example, processing by a digital filter having a size of 1 × 5 is performed.
The following coefficient (11) is used as a filter:

If this process is expressed using the programming language C,
It can be expressed as <List 6>.
[0071]

Here, Table In in the function argument of the function filtering (Table Out, Table In) is a correction gradation curve (that is, LUT) before digital filter processing, and Table Out is correction after digital filter processing. It is a gradation curve.
As a result, the aforementioned E2 becomes a gradation conversion table E3 with smaller gradation skip.
[0072]

  Next, when only one type of gradation conversion table is used in the image processing unit, the gradation conversion table (image processing printer γ correction circuit 109) of the image processing unit and the gradation conversion table of the image forming unit are used. The gradation conversion table obtained by combining the image forming printer γ correction circuit 112 may be set in the printer γ correction circuit 112 of the image forming unit, and “through” may be set in the image processing printer γ correction circuit. Is possible (FIG. 33).).In the case of copier, when conversion is performed using two gradation conversion tables, that is, the gradation conversion table included in the image processing means and the gradation conversion table of the printer unit, an output for adjacent input signals of the gradation conversion table is performed. The difference in values may be large. In the case of a non-conversion (through) gradation conversion table, the difference between adjacent input values is 1 (the output value for the input value 0 is 0, the output value for the input value 1 is 1, and similarly) Since the output value for the input value 255 is 255, the difference between the output values is always 1). The difference is large when it is larger than 1.
[0073]
  When such a gradation conversion table is passed twice, the difference between the output values may be larger than that for one conversion. In such a case, there may be a problem that an image whose density difference is clearly visible, that is, a so-called pseudo contour appears even for an image whose gradation should be smooth. Accordingly, in (STEP 251) of FIG. 31, the gradation conversion table set in the image forming printer γ correction circuit 112 is not a through gradation conversion table, and the following gradation having substantially the same function is provided. You can also use a conversion tableYes.

[0074]
【The invention's effect】
  According to the present invention, a gradation conversion table is provided in each of the image forming unit and the image processing unit, thereby improving color reproduction and ensuring image quality stability.Image forming meansWhenImage reading meansCan also be used as independent devices. Also,Image reading meansWhenImage forming meansBy dividing the gradation conversion table withImage forming meansVariations due to machine differencesImage forming meansThe tone conversion table ofImage reading means・ In the gradation conversion table of the image processing unitImage reading meansBy setting the optimal parameters for each image quality mode,Image reading meansWhenImage forming meansIt is possible to increase the independence of the system and to guarantee the necessary functions. Also, for each mode such as text, photo, map, etc.Of image forming meansBy preparing a gradation table, it is possible to obtain an image more faithful to the original. Also, print dataImage forming meansInput to the image signal switching deviceImage forming meansConverted by the gradation conversion table ofImage forming meansCorrect and use the machine-specific variations.Image forming meansBy printing without depending onImage forming meansEven a single image forming deviceImage reading meansIt can also be used as an image forming apparatus having
[0075]
  In addition, by using the detection result of the optical detection sensor and the detection result of the surface potential sensor together,Image forming meansThe gradation conversion table (Procon γ) is corrected with higher accuracy, thereby correcting the density fluctuation, and always reproducing a good density and obtaining an image faithful to the original. Also, among the image conversion tables divided into several steps, one step is in the printer and has γ for each image quality mode, and γ can be switched for each mode, and the slope of γ is adjusted by density setting can do. Also, after the image signal switching means in the image forming apparatusFor external useTone conversion tableWhenCombining with the internal gradation conversion table included in the image processing means,By setting either the external gradation conversion table or the internal gradation conversion table and setting the other to the non-conversion table,Generation of pseudo contours caused by passing the gradation conversion table twice can be prevented. In addition, when a gradation skip is observed in the combined gradation conversion table, by performing smoothing processing, it is possible to prevent the occurrence of a pseudo contour due to the gradation skip in the gradation conversion table.
[0076]
  Further, each of the plurality of gradation conversion tables in the image processing unit is combined with the external gradation conversion table, and the combined gradation conversion table is converted into each of the gradation conversion tables corresponding to the plurality of regions in the image processing apparatus. By setting to, different gradation conversion tables can be used corresponding to each of a plurality of areas in the document, and the occurrence of pseudo contours can be prevented. Also,For external useA plurality of gradation conversion tables and a plurality of images included in the image processing meansinternalThe tone conversion table obtained by combining the tone conversion table forOutsideSet in the grayscale conversion tableMultiple insideSet in the grayscale conversion table forFor external useTone conversion table orFor internal useBy setting a non-conversion table (through) in the gradation conversion table, it is possible to prevent the setting of a pseudo contour due to conversion using two gradation conversion tables.
[Brief description of the drawings]
FIG. 1 is a block diagram of an image processing unit in the copying machine shown in FIG.
FIG. 2 is a configuration diagram of an embodiment of a copying machine according to the present invention.
FIG. 3 is an explanatory diagram of a control system in the copying machine shown in FIG. 2;
FIG. 4 is an explanatory diagram of a gradation conversion table switching operation in the copying machine shown in FIG. 2;
FIG. 5 is a configuration diagram of a laser modulation circuit in the copying machine shown in FIG. 2;
FIG. 6 is a flowchart of a method of creating a gradation conversion table performed by the image processing printer γ correction circuit.
FIG. 7 is an explanatory diagram of an operation for selecting the entire degree of curvature in STEP 201 of FIG. 6;
FIG. 8 is an explanatory diagram of an operation for selecting the entire degree of curvature in STEP 201 of FIG. 6;
FIG. 9 is an explanatory diagram of an operation for selecting the entire degree of curvature in STEP 201 of FIG. 6;
FIG. 10 is an operation flowchart of automatic gradation correction (ACC) of image density (gradation).
11 is a diagram showing an operation unit in the copying machine shown in FIG. 2. FIG.
12 is a diagram showing a display example of an operation pattern on the liquid crystal screen shown in FIG.
13 is a diagram showing a display example of an operation pattern on the liquid crystal screen shown in FIG.
FIG. 14 is a diagram illustrating a density gradation pattern formed on a transfer material.
15 is a diagram showing a display example of an operation pattern on the liquid crystal screen shown in FIG.
16 is a diagram showing a display example of an operation pattern on the liquid crystal screen shown in FIG.
FIG. 17 is a flowchart of a development characteristic detection method in a gradation conversion table creation method set in an image forming printer γ correction circuit.
18 is an explanatory diagram for forming a density pattern on a photoconductor in the detection method shown in FIG.
FIG. 19 is a quaternary chart showing a method of correcting an image signal.
FIG. 20 is a flowchart of a correction method for compensating for a deviation in development characteristics obtained from actual development characteristics and detection values.
FIG. 21 is an explanatory diagram of a correction method for compensating for a deviation in development characteristics obtained from actual development characteristics and detection values.
FIG. 22 is a flowchart of a method for acquiring a gradation conversion table.
FIG. 23 is a flowchart of a correction method for correcting so that the gradation which has changed over time after the initial setting and after a predetermined time does not change apparently.
FIG. 24 is an explanatory diagram of a gradation correction method that can reduce the storage capacity of the correction table.
FIG. 25 is a flowchart of a gradation correction method that can reduce the storage capacity of the correction table.
FIG. 26 is a diagram showing correction data of RGB signals.
27 is a view showing a display example of an operation pattern on the liquid crystal screen shown in FIG.
FIG. 28 is an explanatory diagram of a gradation conversion table selection operation.
FIG. 29 is a flowchart of a procedure for creating a gradation conversion table when ACC is executed.
FIG. 30 is a flowchart of a method for selecting a gradation conversion table stored in a ROM for creating a corrected gradation curve.
FIG. 31 is a flowchart of a method for synthesizing a gradation conversion table set in the image processing printer γ correction circuit and a gradation conversion table set in the image forming printer γ correction circuit;
FIG. 32 is an explanatory diagram of a method for synthesizing a gradation conversion table set in the image processing printer γ correction circuit and a gradation conversion table set in the image forming printer γ correction circuit.
FIG. 33 shows a gradation obtained by synthesizing the gradation conversion table (image processing printer γ correction circuit) of the image processing unit and the gradation conversion table (image forming printer γ correction circuit) of the image forming unit. FIG. 6 is an explanatory diagram when a conversion table is set in the printer γ correction circuit of the image forming unit and through is set in the image processing printer γ correction circuit.
[Explanation of symbols]
1 ... Copier body, 2 ... Drum,
3 ... Charge charger, 4 ... Laser optical system,
5 ... Black developing device 6, 7, 8 ... Color developing device,
9 ... Intermediate transfer belt, 10 ... Bias roller,
11 ... Cleaning device, 12 ... Static elimination part,
13 ... transfer bias roller, 14 ... belt cleaning device,
15 ... Conveying belt, 16 ... Fixing device,
17 ... Output tray, 18 ... Contact glass,
19 ... exposure lamp, 21 ... reflection mirror,
22 ... imaging lens, 23 ... image sensor array,
30 ... main control unit (CPU), 31 ... ROM,
32 ... RAM, 34 ... Laser optical system controller,
35 ... power supply circuit, 36 ... optical sensor,
37 ... Toner density sensor, 38 ... Environmental sensor,
39 ... Photoconductor surface potential sensor, 40 ... Toner supply circuit,
41 ... Intermediate transfer belt drive unit, 42 ... Operation unit,
44 ... agent stirring member, 45 ... developing sleeve,
101: Shading correction circuit, 102: Area processing circuit,
103: Scanner γ conversion circuit, 104: Image separation circuit,
105: MTF filter, 106: Color conversion UCR processing circuit,
107 ... zoom circuit 108 ... image processing circuit,
109: Image processing printer γ conversion circuit, 110: Gradation processing circuit,
111, 123 ... interface I / F selector
112... Printer γ conversion circuit for image forming unit,
113 ... Printer,
117 ... System controller, 118 ... BUS,
120 ... color scanner,
121, 122 ... pattern generation circuit,
151 ... Lookup table,
152 ... Pulse width modulation circuit, 153 ... Power modulation circuit,
154 ... Laser diode, 155 ... Photo detector,
156: LCD screen,

Claims (6)

An image reading unit that optically scans and reads a document image placed at a reading position, an image processing unit that processes an image signal from the image reading unit and outputs it as an output image signal, and an image carrier according to the output image signal An image writing means for writing information on the body, an image switching means for switching an output image signal from the image processing means and an output image signal from an external device to output to the image writing means, and on the image carrier An image forming apparatus having an image forming unit that visualizes the information with a plurality of color developers and forms an image on a transfer material,
An internal gradation conversion table used for gradation conversion of the output image signal from the image processing means;
Image signal generating means for generating a plurality of gradation patterns on the image carrier;
Surface potential detecting means for detecting a surface potential of a gradation pattern formed on the image carrier by the image signal generating means;
Optical detection means for detecting the amount of developer in a gradation pattern that has been subjected to gradation conversion of the output image signal from the external device by switching by the image switching means and visualized on the image carrier;
It is created or selected based on the detection result of the surface potential detection means and the detection result of the optical detection means, and gradation conversion is performed using the internal gradation conversion table by switching by the image switching means. An external gradation conversion table used for gradation conversion before the output image signal from the image processing means or the output image signal from the external device is output to the image writing means;
An image forming apparatus comprising: An image reading unit that optically scans and reads a document image placed at a reading position, an image processing unit that processes an image signal from the image reading unit and outputs it as an output image signal, and an image carrier according to the output image signal An image writing means for writing information on the body, an image switching means for switching an output image signal from the image processing means and an output image signal from an external device to output to the image writing means, and on the image carrier An image forming apparatus having an image forming unit that visualizes the information with a plurality of color developers and forms an image on a transfer material,
Image signal generating means for generating a plurality of gradation patterns on the image carrier;
Storage means for storing gradation target data corresponding to a read signal of a gradation pattern formed on the image carrier by the image signal generating means;
It is created or selected based on the gradation pattern read signal read by the image reading means and the gradation target data stored in the storage means, and the output image signal from the image processing means is gradation converted. An internal tone conversion table to be used for
The output image signal from the image processing unit or the output image signal from the external device, which has been subjected to tone conversion using the internal tone conversion table by the switching by the image switching unit, is output to the image writing unit. An external gradation conversion table used for gradation conversion before output;
An image forming apparatus comprising:   When only the image signal from the image processing unit is output by the image switching unit, the combined gradation conversion table obtained by combining the external gradation conversion table and the internal gradation conversion table is the 3. The image forming apparatus according to claim 1, wherein one of the external gradation conversion table and the internal gradation conversion table is set, and the other is set as a non-conversion table.   The image forming apparatus according to claim 3, wherein smoothing processing is performed on a combined gradation conversion table obtained by combining the external gradation conversion table and the internal gradation conversion table.   A combined gradation conversion table obtained by combining the external gradation conversion table and a plurality of gradation conversion tables included in the internal gradation conversion table is used as each gradation of the internal gradation conversion table. The image forming apparatus according to claim 3, wherein the image forming apparatus is set in a conversion table.   A composite gradation conversion table obtained by combining the external gradation conversion table and the internal gradation conversion table is set in either the external gradation conversion table or the internal gradation conversion table. 4. The image forming apparatus according to claim 3, wherein a non-conversion table is set for the other.

Images