JP2004249677A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of achieving good graininess with a value of "graininess degree" of 0.30 or below even if an error diffusion method excellent in the sharpness of an image or writing positional precision and hard to cause a moire is employed. <P>SOLUTION: When the lengths of one pixel calculated based on resolving powers in the main scanning direction and sub-scanning direction of irradiated light are respectively set to Lp1 and Lp2 and the beam diameters in the main scanning direction and sub-scanning direction of irradiated light on a photosensitive drum 1 are set to D1 and D2, the relations of D1<Lp1 and D2<Lp2 are satisfied and the volume average particle size of toner in a developing agent used in a developing device 4 is 5.5 μm or below. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００２６】
そして、次に入力画像データｄ_ｘ、ｙと補正値Ｅ_ｘ、ｙが加算部１０３ａで加算され、補正画素データＤ_ｘ、ｙが出力される。次に、量子化部１０３ｂによって補正画素データＤ_ｘ、ｙが量子化される。より具体的には、量子化部１０３ｂは実施例１では２であり、出力用画像データは０または１の値をとる。
そして、誤差演算部１０３ｃは補正画素データＤ_ｘ，ｙと出力用画像データＯ_ｘ、ｙおよびあらかじめ定められた値Ｂ（実施例１では量子化数が２であるため、２５５）に応じて注目画素における誤差ｅ_ｘ、ｙが次式により算出される。
【数２】

【００２７】
実施例１では、量子化数は２であるが別の値であっても全くかまわない。通常の多値誤差最小法では、Ｏ_ｘ，ｙ×Ｂの値は、０から入力画像データの最大値（たとえば２５５）の間で出力画像データＯ_ｘ、ｙの量子化数に応じて等分された値とされるが、任意の値としてもよい。その場合には出力画像画像データＯ_ｘ、ｙに応じた値を予め定められたルックアップテーブルなどが用いられる。
【００２８】
上述のようにして画像処理部１０において処理を施された結果は、出力用画像データとして次工程であるビデオ信号処理部１２へと送られる。ビデオ信号処理部１２では前述の出力用画像データを受け取り、発光部の数（実施例１では２ＬＤ）分のデータをラインメモリ上に記憶し、前述のポリゴンミラー３０６の回転に合わせて（いわゆる同期信号）、光変調信号としてＬＤ駆動ドライバー部へと出力する。このＬＤ駆動ドライバー部がＬＤユニット３０１の２個のＬＤを同時に駆動することによって、感光体ドラム表面上に所望の潜像を形成していく。
【００２９】
実施例１では、上述のように擬似中間調処理方法は誤差拡散法である。誤差拡散法の詳細は上述の通りである。この誤差拡散法では、擬似中間調処理後の出力用画像データは、周期的な構造を持たない。誤差拡散法は、一般的にＦＭスクリーンと呼ばれる擬似中間調処理方法のひとつであり、ドットの密度（画像の空間周波数）を変化させて、画像の階調を再現している。このため、一定の周期構造を持たないといった特徴がある。
【００３０】
次に、実施例１の画像形成装置で用いたトナーについて説明する。実施例１で使用するトナーは、以下の方法で作製したいわゆる重合トナーである。
【００３１】
〜樹脂微粒子エマルションの合成〜
撹拌棒および温度計をセットした反応容器に、イオン交換水６８３部、メタクリル酸エチレンオキサイド付加物硫酸エステルのナトリウム塩（エレミノールＲＳ−３０：三洋化成工業製）１１部、スチレン８３部、メタクリル酸８３部、アクリル酸ブチル１１０部、過硫酸アンモニウム１部を仕込み、４００回転／分で１５分間撹拌したところ、白色の乳濁液が得られた。加熱して、系内温度８０℃まで昇温し５時間反応させた。さらに、１％過硫酸アンモニウム水溶液３０部を滴下して加え、８０℃で７時間熟成してビニル系樹脂（スチレン−メタクリル酸−アクリル酸ブチル−メタクリル酸エテレンオキサイド付加物硫酸エステルのナトリウム塩の共重合体）の水性分散液［微粒子分散液１］を得た。［微粒子分散液１］をＬＡ−９２０で測定した体積平均粒径は、０．０９μｍであった。［微粒子分散液１］の一部を乾燥して樹脂分を単離した。該樹脂分のＴｇは５８℃であった。
【００３２】
〜水相の調整〜
イオン交換水１０００部、［微粒子分散液１］８３部、ドデシルジフェニルェーテルジスルホン酸ナトリウムの４８．５％水溶液（エレミノールＭＯＮ−７：三洋化成工業製）３７部、酢酸エチル９０部を混合撹拌し、乳白色の液体を得た。これを［水相１］とする。
【００３３】
〜低分子ポリエステルの合成〜
冷却管、撹拌機および窒素導入管の付いた反応容器中に、ビスフェノールＡエチレンオキサイド２モル付加物２２９部、ビスフェノールＡプロピレンオキサイド３モル付加物５２９部、テレフタル酸２０８部、アジピン酸４６部およびジブチルチンオキサイド２部を入れ、常圧で２３０℃で８時間反応し、さらに１０〜１５ｍｍＨｇの減圧で５時聞反応した後、反応容器に無水トリメリット酸４４部を入れ、１８０℃、常圧で２時間反応し、［低分子ポリエステル１］を得た。［低分子ポリエステル１〕は、数平均分子量２５００、重量平均分子量６７００，Ｔｇ４３℃であった。
【００３４】
〜イソシアネート基を有するプレポリマーの合成〜
冷却管、撹拌機および窒索導入管の付いた反応容器中に、ビスフェノールＡエチレンオキサイド２モル付加物６８２部、ビスフェノールＡプロピレンオキサイド２モル付加物８１部、テレフタル酸２８３部、無水トリメリツト酸２２部およびジブチルチンオキサイド２部を入れ、常圧で２３０℃で８時間反応し、さらに１０〜１５ｍｍＨｇの減圧で５時間反応した［中間体ポリエステル１］を得た。［中間体ポリエステル１］は、数平均分子量２１００、重量平均分子量９５００、Ｔｇ５５℃、酸価０．５、水酸基価５１であった。
次に、冷却管、撹拌機および窒素導入管の付いた反応容器中に、［中間体ポリエステル１］４１０部、イソホロンジイソシアネート８９部、酢酸エチル５００部を入れ１００℃で５時間反応し、［プレポリマー１］を得た。［プレポリマー１］の遊離イソシアネート重量％は、１．５３％であった。
であった。
【００３５】
〜ケチミンの合成〜
撹拌棒および温度計をセットした反応容器に、イソホロンジアミン１７０部とメチルエチルケトン７５部を仕込み、５０℃で５時間反応を行い、［ケチミン化合物１］を得た。［ケチミン化合物１］のアミン価は４１８であった。
【００３６】
〜顔料マスターバッチの調整〜
水１２００部、カーボンブラック（Ｐｒｉｎｔｅｘ６０，デクサ社製）５４０部、［低分子ポリエステル１］１２００部を加え、ヘンシェルミキサー（三井鉱山社製）で混合し、混合物を２本ロールを用いて１３０℃で４５分混練後、圧延冷却しパルペライザーで１ｍｍφ以下に粉砕し［マスターバッチ１］を得た。
【００３７】
〜油相の作成〜
撹拌棒および温度計をセットした容器に、［低分子ポリエステル１］３７８部、合成エステルワックス１１Ｏ部、ＣＣＡ（サリチル酸金属錯体Ｅ−８４：オリエント化学工業）２２部、酢酸エチル９４７部を仕込み、撹拌下８０℃に昇温し、８０℃のまま５時間保持した後、１時問で３０℃に冷却した。次いで容器に［マスターバッチ１］５００部、酢酸エチル５００部を仕込み、１時間混合し［原料溶解液１］を得た。
この［原料溶解液１］１３２４部を容器に移し、ビーズミル（ウルトラビスコミル、アイメックス社製）を用いて、送液速度１ｋｇ／ｈｒ、ディスク周速度６ｍ／秒、０．５ｍｍジルコニアビーズを８０体積％充填、３パスの条件で、カーボンブラック、ＷＡＸの分散を行った。次いで、［低分子ポリエステル１］の６５％酢酸エチル溶液１３２４部加え、上記条件のビーズミルで１パスし、［顔料・ＷＡＸ分散液１］を得た。［顔料・ＷＡＸ分散液１］の固形分濃度（１３０℃、３０分）は５０％であった。
【００３８】
〜乳化⇒脱溶剤〜
［顔料・ＷＡＸ分散液１］６５０部、［プレポリマー１］を１４０部、［ケチミン化合物１］６．０部を容器に入れ、ＴＫホモミキサー（特殊機化社製）で５，０００ｒｐｍで１分間混合した後、容器に［水相１］１２００部を加え、ＴＫホモミキサーで、回転数１３，０００ｒｐｍで２０分間混合し［乳化スラリー１］を得た。
撹拌機および温度計をセットした容器に、［乳化スラリー１］を投入し、３０℃で８時間脱溶剤した後、４０℃で８時間熟成を行い、［分散スラリー１］を得た。
【００３９】
〜洗浄⇒乾燥〜
［乳化スラリー１］１００部を減圧濾過した後、
▲１▼：濾過ケーキにイオン交換水１００部を加え、ＴＫホモミキサーで混合（回転数１２，０００ｒｐｍで１０分間）した後濾過した。
▲２▼：▲１▼の濾過ケーキに１０％水酸化ナトリウム水溶液１ＯＯ部を加え、ＴＫホモミキサーで混合（回転数１２，０００ｒｐｍで３０分間）した後、減圧濾過した。
▲３▼：▲２▼の濾過ケーキに１０％塩酸１００部を加え、ＴＫホモミキサーで混合（回転数１２，０００ｒｐｍで１０分間）した後濾過した。
▲４▼：▲３▼の濾過ケーキにイオン交換水３００部を加え、ＴＫホモミキサーで混合（回転数１２，０００ｒｐｍで１０分間）した後濾過する操作を２回行い［濾過ケーキ１］を得た。
この［濾過ケーキ１］を循風乾燥機にて４５℃で４８時間乾燥し、目開き７５μｍメッシュで篩った後、トナー粒子１００部に疎水性シリカ（ヘキサメチルジシラザン表面処理品、比表面積：２００ｍ^２／ｇ）０．５部と、疎水化ルチル型酸化チタン（イソブチルトリメトキシシラン表面処理品、平均一次粒子径：０．０２μｍ）０．５部をヘンシェルミキサーにて混合して、トナーＡを得た。このトナーの体積平均粒径は５．４３μｍ、Ｔｇは４６℃、樹脂成分のＴＨＦ不溶分は１２％であった。
【００４０】
以上の方法で作製したトナーの粒径は、コールターエレクトロニクス社製の粒度測定器「コールターカウンターＴＡＩＩ」を用い、アパーチャー径１００μｍで測定した。トナーの体積平均粒径および個数平均粒径は上記粒度測定器により求めたのもである。実施例１で作成したトナーの体積平均粒径は５．５μｍであった。また、このトナー中の顔料の樹脂に対する質量比率は６．０％であった。
【００４１】
実施例１のトナー作製方法は上記の通りであるが、本発明は、他の方法で作成したトナーを使用する場合にも適用できるものである。例えば、上記方法のほか分散重合法や、粉砕法によって作製したトナーであっても構わない。
【００４２】
次に、実施例１の画像形成装置で用いた感光体ドラム１について説明する。
実施例１の感光体ドラム１は、φ９０ｍｍのアルミニウムシリンダー上に下記組成の下引き層塗工液、電荷発生層塗工液、および電荷輸送層塗工液を、浸漬塗工によって順次塗布、乾燥して作成したものである。この作成方法により、膜厚３．５μｍの下引き層（ＵＬ層）、０．２μｍの電荷発生層（ＣＧ層）、２０μｍの電荷輸送層（ＣＴ層）を形成している。なお、膜厚はフィッシャースコープ社製の膜厚計で測定したものである。
【００４３】
〔下引き層塗工液〕
・二酸化チタン粉末：４００部
・メラミン樹脂：６５部
・アルキッド樹脂：１２０部
・２−ブタノン：４００部
〔電荷発生層用塗工液〕
クロロガリウムフタロシアニン顔料：２部
ポリビニルブチラール（エスレックＢＭ−１：積水化学製）：１．０部
シクロヘキサノン：３０部
メチルエチルケトン：７０部
〔電荷輸送層塗工液〕
・ポリカーボネート（パンライトＣ−１４００、帝人化成製）：６部
・下記化１の構造式の電荷輸送物質：４部
（キャリア移動度：１×１０^−５ｃｍ^２・Ｖ^−１・ｓｅｃ^−１）
・テトラヒドロフラン：５０部
【化１】

【００４４】
実施例１の感光体ドラム１の仕様は上記の通りであるが、これに限定されるものでない。他の材料を使用した感光体ドラムであっても良いし、実施例１のような積層型感光体でなく単層型の感光体であっても良い。
【００４５】
次に、出力画像の評価について説明する。２５６段のパッチ（全階調）のうちのあらかじめ決定した１７段を使用して、この１７段のパッチ（１５ｍｍ四方）をスキャナーで読み込み、下記で説明する方法によって粒状度を計算・導出した。この「粒状性」は、「均一であるべき画像がどれだけざらついているかを表す主観評価値」と定義される。この主観的な評価値である粒状性を客観的に表した量が粒状性の評価尺度であり、「粒状度」である。この粒状度として標準化されているものとしてＲＭＳ粒状度があり、ＡＮＳＩ ＰＨ−２．４０−１９８５で標準化され、次式で表される。式中のＤｉは濃度分布、Ｄは平均濃度（Ｄ＝（１／Ｎ）・ΣＤｉ）である。
【数３】

【００４６】
このほか、次式で示す画像の濃度変動のパワースペクトラムであるＷｉｎｅｒ Ｓｐｅｃｔｒｕｍを用いた粒状度が定義されている。ＸｅｒｏｘのＤｏｏｌｅｙとＳｈａｗはＷｉｎｅｒ Ｓｐｅｃｔｒｕｍを適用し、視覚の空間周波数特性（Ｖｉｓｕａｌ Ｔｒａｎｓｆｅｒ Ｆｕｎｃｔｉｏｎ：ＶＴＦ）とカスケードした後、積分した値を粒状度（ＧＳ）とした（詳細は、Ｒ．Ｐ．Ｄｏｏｌｅｙ，Ｒｓｈａｗ：Ｎｏｉｓｅ Ｐｅｒｃｅｐｔｉｏｎ ｉｎ Ｅｌｅｃｔｒｏｐｈｏｔｏｇｒａｐｈｙ，Ｊ．Ａｐｐｌ．Ｐｈｏｔｏｇｒ．Ｅｎｇ．，５，４（１９７９），ｐｐ１９０−１９６参照）。
【数４】

【００４７】
本実施例における「粒状度」は、このＤｏｏｌｅｙとＳｈａｗによる粒状度をさらに発展させ、次式によって定義されるものである。
【数５】

【００４８】
この場合には、画像の濃度Ｄではなく、明度Ｌ＊を使用する。後者の方が色空間のリニアリティーに優れ、カラー画像への適応性も優れる点が特徴である。以下で記載する粒状度は、この数５で示す式によって定義された粒状度である。
粒状度はその定義からして画像のノイズ特性を表している。出力画像の粒状度を上述の手法によって測定することによって、画像のノイズ特性（ざらつき）を数値化することが可能である。粒状度の数値はその定義からも分かるように、ざらつきが良好である場合には値が小さく、ざらつきが悪くなるに従って値が大きくなる。本実施例では、出力画像をスキャナー（ＦＴ−Ｓ５０００：大日本スクリーン社製）で読み込んだ後に、上記数５で示す計算式に基づき粒状度の計算を行った。
【００４９】
図９は粒状度の数値化の一例を示すグラフである。横軸が明度、縦軸が粒状度となっている。粒状度は明度ごとに与えられるものであり、明度を１７水準（１７段のパッチ）ごとに粒状度が数値化されている。これら１７段のパッチのうち明度値（横軸）が８０、７０、６０、５０、４０にそれぞれもっとも近い５パッチを選択した。そして、この５パッチの粒状度の値を算術平均して算出した値を、その画像出力条件での粒状度として定義することにした。以後、本実施例における粒状度は、この算術平均により計算された粒状度である。
【００５０】
また、画像の出力の実験は、本実施例１で記載した構成をもつ実験機（リコー社製：ＩＭＡＧＩＯ Ｃｏｌｏｒ ５１００）をベースにして改造した実験機）により行った。画像出力及び粒状度の測定の結果、実施例１の構成の画像形成装置での粒状度の値は、０．２０であった。粒状度の値としては、０．３０以下であれば、印刷をはじめとする電子写真方式以外のハードコピー機器と比較しても遜色なく、高画質の要件を十分満たすと考え、粒状度が０．３０以下であることが画質上の制約であると考えた。
このように実施例１の画像形成装置では、擬似中間調処理方法として誤差拡散法をしても、出力画像の粒状性が上述のように良好である。
【００５１】
（実施例２、３）
実施例２及び３では、実施例１と同じ画像形成装置において、誤差拡散処理工程で使用するウェイトマトリクスのみを下記のように変えた。実施例２及び実施例３で使用したウェイトマトリックスについてはそれぞれ図７及び図８に示す。また、各実施例１、２及び３における解像度は６００ｄｐｉ（主走査方向）×６００ｄｐｉ（副走査方向）である。
【００５２】
実施例１：誤差拡散Ａ（図６のウェイトマトリクスＡ）
実施例２：誤差拡散Ｂ（図７のウェイトマトリクスＢ）
実施例３：誤差拡散Ｃ（図８のウェイトマトリクスＣ）
【００５３】
表１は、各実施例１、２及び３における粒状度の評価結果である。いずれのウェイトマトリクスにおいても、粒状性が良好である画像形成装置を実現することができる。
【表１】

【００５４】
（比較例１、２、３）
比較例１、２及び３では、実施例１と同じ画像形成装置において、感光体ドラム１上のビーム径Ｄ１，Ｄ２のみを下記のように変えた。ビーム径の変更はアパーチャーの変更により行い、調整後のビーム径をビームスキャンで測定して確認した。
【００５５】
比較例１：５０μｍ（主走査方向）×５０μｍ（副走査方向）
比較例２：６０μｍ（主走査方向）×６０μｍ（副走査方向）
比較例３：７０μｍ（主走査方向）×７０μｍ（副走査方向）
【００５６】
表２は、各比較例１、２及び３における粒状度の評価結果である。いずれの比較例においても、粒状度が０．３以上であった。
【表２】

【００５７】
実施例１〜３及び比較例１〜２はいずれも、解像度６００ｄｐｉの２値誤差拡散であり、解像度に基づく１画素あたりの主走査方向及び副走査方向の長さＬｐ１，Ｌｐ２は４２．３μｍである。ここで、図１に示すように感光体ドラム表面上におけるレーザビームＬＢの主走査方向及び副走査方向のビーム径をそれぞれＤ１及びＤ２としたときに、下記の数６に示す条件式の関係を満たすことが、粒状度を悪化させない条件である。表２の結果から、上記条件を満たすことによって粒状度の悪化を防止することができるメカニズムについては後で説明する。
【数６】
Ｄ１＜Ｌｐ１
Ｄ２＜Ｌｐ２
【００５８】
次に、画像形成装置の解像度を１２００ｄｐｉにして画像の出力を行った結果を示す。解像度を６００ｄｐｉから１２００ｄｐｉに変更した場合には、１画素あたりの主走査方向及び副走査方向の長さは、Ｌｐ１＝Ｌｐ２＝２１．２μｍである。解像度を１２００ｄｐｉに変更して画像の出力を行うためには、実施例１において画素クロックの周波数を７８．３ＭＨｚ、ポリゴンミラー３０６の回転数を３４７２４ｒｐｍで、ＬＤの数を２倍にすることによって実現することができる。合計４つのＬＤが必要となるため、光源のレイアウトの制約から１チップ上に４つのＬＤを配列して形成された４チャンネルＬＤアレイを使用した。
【００５９】
（実施例４）
実施例４では、アパーチャーの交換により感光体ドラム１上でのビーム径を２０μｍ（Ｄ１：主走査方向）×２０μｍ（Ｄ２：副走査方向）とした。解像度は上述の変更により１２００ｄｐｉとした。
【００６０】
（比較例４）
比較例４では、アパーチャーの交換により感光体ドラム１上でのビーム径を３０μｍ（Ｄ１：主走査方向）×３０μｍ（Ｄ２：副走査方向）とした。解像度は上述の変更により１２００ｄｐｉとした。
【００６１】
（比較例５）
比較例５では、アパーチャーの交換により感光体ドラム１上でのビーム径を４０μｍ（Ｄ１：主走査方向）×４０μｍ（Ｄ２：副走査方向）とした。解像度は上述の変更により１２００ｄｐｉとした。
【００６２】
表３は、解像度が１２００ｄｐｉである実施例４及び比較例４、５における粒状度の評価結果である。表３からわかるように、やはりビーム径が大きくなると（具体的には３０（主）×３０（副）μｍ以上になると）、粒状度の値が著しく悪化することが分かる。１２００ｄｐｉにおいても６００ｄｐｉの場合と同じく、前述の数６で示す条件式の関係を満たすことが、粒状度を悪化させない条件の一つであることがわかる。
【表３】

【００６３】
次に、主走査方向及び副走査方向の解像度に基づく１画素あたりの長さをそれぞれＬｐ１，Ｌｐ２とし、感光体ドラム表面上におけるレーザビームＬＢの主走査方向及び副走査方向のビーム径をそれぞれＤ１及びＤ２としたときに、Ｄ１＜Ｌｐ１、かつＤ２＜Ｌｐ２の関係を満たす場合に、擬似中間調処理方法として誤差拡散法を用いた場合でも粒状性の悪化が起こらない理由を説明する。詳細の理由については推測の域をでないが、発明者らの実験結果に基づく経験的な説明である。
【００６４】
誤差拡散法では、擬似中間調処理後の画像データは前述のように周期的な構造を持たない。図１０は誤差拡散処理を施した画像データ（画像は８０／２５５に相当するパッチの一部を１０×１０画素抽出）であるが、誤差拡散法による擬似中間調処理では、図１０のように書きこみ画素の配置が非周期的であるため、隣接する書きこみ画素との位置関係もまちまちである。周囲に書き込まれた画素がまったくない場合と、多くの書き込まれた画素と隣接している場合とが混在している。
【００６５】
図１０のような画像データに対応して、実際にレーザービームによって書き込みが行われるのであるが、レーザービームのビームスポット径によって、形成される静電潜像の形は変化する。ビームスポット径が画像データの１画素よりも大きな場合（Ｄ１＞Ｌｐ１、Ｄ２＞Ｌｐ２）である場合には、書き込みを行っていない画素の部分まで同時に露光してしまう。このような部分での電位は、未露光部と露光部との中間的な電位となる。一方、ビームスポット径が画像データの１画素よりも小さい場合（Ｄ１＞Ｌｐ１、Ｄ２＞Ｌｐ２）には、現実的には感光体の電荷輸送層などで潜像が面方向に拡散するため、上述のような中間的な電位となってしまう部分生じる。この中間的な電位となる領域は、図１１に示すようにビームスポット径が画像データの１画素よりも大きな場合に比べて各段に少ない。
【００６６】
また、電子写真方式の画像形成装置では、現像工程において周波数依存性を持つことが避けられないため、潜像の周波数ごとに異なるトナー付着量となる。現像工程での空間周波数依存性の結果、注目した画素に付着するトナーの量は、周囲に存在する書き込みを行われた画素によって変化する。このトナーの付着量が強調されたり鈍化されたりすることが、注目画素の周囲に存在する書き込みを行われた画素によって決定されるのである。このような周波数依存性は、上述の未露光部と露光部との中間的な電位となる領域が多くなるにしたがって、その影響が無視できなくなる。つまり、この中間的な電位となっている部分にトナーが付着するか付着しないかは、周囲の状況によって決定される。誤差拡散法では、周囲の書き込み状況は図１０のように一定ではないため、このような中間的な電位部にトナーが付着するか付着しないかによって、巨視的に見た場合には低周波な濃度変動が発生しているように見え、粒状性の悪い画像となると予想している。
【００６７】
これに対して、ビームスポット径が画像データの１画素よりも小さい場合（主走査方向：Ｄ１＜Ｌｐ１、副走査方向：Ｄ２＜Ｌｐ２）には、中間的電位となってしまう部分が少ないために、粒状性の悪い画像にはならないのである。
【００６８】
（比較例６、７、８）
次に、上記実施例１〜４の画像形成装置とディザ法を用いた画像形成装置との比較実験の結果について説明する。比較例６〜９の画像形成装置で用いた擬似中間調処理は次のとおりである。
比較例６：２８０ｌｐｉディザ
（スクリーン各４５°、副走査方向への周期構造：あり）
比較例７：２４０ｌｐｉディザ
（スクリーン角５３°、副走査方向への周期構造：あり）
比較例８：２１２ｌｐｉディザ
（スクリーン各４５°、副走査方向への周期構造：あり）
比較例９：１７０ｌｐｉディザ
（スクリーン各４５°、副走査方向への周期構造：あり）
【００６９】
擬似中間調処理法として誤差拡散法を使用した実施例１〜４の画像形成装置においては、粒状度が十分良好であり、なおかつ次の▲１▼〜▲３▼のような特長を有する。
すなわち、▲１▼ディザ法に比べて鮮鋭性に優れた画像の実現が可能である。また、▲２▼位置精度が低いことによる異常画像の発生が少ない。実施例１に記載した実験装置では、２つのＬＤを搭載しているが、２ＬＤの波長差によるいわゆる倍率誤差に偏差が生じるため、主走査方向に最大で３０μｍ程度の書き込み位置ずれが生じている。しかしながら実施例１の構成ではこのような書き込み位置ずれの影響が出力画像に現れることはなかった。これに対して、ディザ法を用いた場合には、わずかながら主走査方向につらなるライン上の異常画像がみられた。更に、▲３▼周期的構造を持つような入力データを入力した場合であってもモアレが発生しなかった。
【００７０】
これに対して、上記比較例６〜８では、表４に示すように良好な粒状性の実現と上記▲１▼〜▲３▼のすべてを両立する条件は見つからなかった。なお、表４の画質評価は目視評価によるものである。
【表４】

【００７１】
（実施例５、６、比較例１０、１１）
次に、実施例１の画像形成装置においてトナーの仕様を変更した場合の画像出力結果について説明する。トナーの体積平均粒径の測定は、前述のとおり、コールターカウンターによって測定したものである。実施例５及び比較例１０、１１で使用したトナーは次のとおりである。
比較例１０：実施例１とほぼ同処方で重合法により作製したトナー（体積平均粒径：６．９μｍ）
実施例５：粉砕法により作製したトナー（体積平均粒径：５．５μｍ）
比較例１１：粉砕法により作製したトナー（体積平均粒径：６．５μｍ）
【００７２】
表５は、実施例５及び比較例１０、１１における粒状度の評価結果である。表５から明らかなように、体積平均粒径が５．５μｍの粉砕法で作製したトナーの場合に、粒状度の値が０．２０となり、良好な粒状性が得られた。これに対し、トナーの体積平均粒径が大きい（６．５μｍ、６．９μｍ）場合には、トナーの作成方法の種類にかかわらず粒状性を良好にすることができなかった。
【表５】

【００７３】
更に、実施例６として、実施例１と同じ画像形成装置において、平均体積粒径が４．５μｍの重合法で作製したトナーを用いて画像を出力し、粒状度を評価した。その結果、表６に示すように粒状度の値が０．１９となり、良好な粒状性が得られた。
【表６】

【００７４】
これらの実施例５、６及び比較例１０、１１の結果と、前述の実施例１の結果から、トナーの体積平均粒径が少なくとも５．５μｍ以下である場合において粒状性が良好になることがわかる。しかも、その粒状性は、トナーの作製法（重合法、粉砕法）には依存しないがわかる。
【００７５】
（実施例７）
次に、実施例１の画像形成装置において、トナー中の樹脂に対する顔料（色材）の質量比率（以下「顔料比率」という。）を変更した実施例７における画像出力結果を説明する。画像の評価項目は前述の粒状度であり、擬似中間調処理は実施例１で用いたウェイトマトリクスを使用した解像度６００ｄｐｉの２値誤差拡散処理である。
【００７６】
図１２は、トナー中の顔料比率を３〜１４％の水準で振ったトナーを用意して、それぞれのトナーについてビーム径を変更して画像出力を行い、粒状度を測定した結果である。このとき使用した擬似中間調処理は実施例１でのウェイトマトリクスを使用した誤差拡散法である。図１２から、顔料比率が３〜８％の範囲であり、且つ感光体ドラム１上の主・副走査方向のビーム径（Ｄ１、Ｄ２）がそれぞれ４０μｍ以下の場合に、今回設定した粒状度０．３以下であるという条件を満足することがわかる。
【００７７】
（実施例７，８、比較例１２）
次に、実施例１の画像形成装置において、感光体ドラム１の仕様を変更した場合のの画像出力結果を説明する。感光体ドラム１の電荷輸送層（ＣＴ層）の膜厚ｄの測定は、前述のとおりフィッシャー社製の膜厚測定器によるもである。
実施例７：電荷輸送層の膜厚ｄを２５μｍとした感光体ドラム
実施例８：電荷輸送層の膜厚ｄを３５μｍとした感光体ドラム
比較例１２：電荷輸送層の膜厚ｄを５０μｍとした感光体ドラム
【００７８】
表７は、実施例７，８及び比較例１２における粒状度の評価結果である。表７から分かるように、感光体ドラム１の電荷輸送層の膜厚ｄがｄ＜Ｄ１、かつｄ＜Ｄ２の条件を満たす場合に、中間調処理として誤差拡散法を使用した場合でも粒状性が悪化しない条件となっていることがわかる。ここで、Ｄ１及びＤ２はそれぞれ、感光体ドラム１上の主走査方向及び副走査方向のビーム径（直径）である。
【表７】

【００７９】
なお、以上示した実施例１〜８及び比較例１〜１２における主要な実験条件と評価結果をまとめたものを、表８に示す。
【表８】

【００８０】
（実施例９）
次に、実施例１の画像形成装置における露光装置（光書き込みユニット）３の構成を変更した実施例９について説明する。
図１３は、本実施例９に係る画像形成装置における露光装置３の概略図である。この露光装置３のＬＤユニット以外の構成は実施例１と同じである。実施例９では、２つの１ｃｈタイプのＬＤユニット３１０を組み合わせて使用している。各ＬＤユニット３１０のＬＤから出射したビーム状のレーザー光をミラー３１１及びハーフミラー３１２を使用して重ね合わせている。
本実施例９のように複数のＬＤユニット３１０を搭載した場合にも、やはり各ＬＤの特性の違いなどにより長期間に渡って各ＬＤの光量を一致させることは難しい。特に実施例１のＬＤアレイからなる２ｃｈタイプのＬＤユニット３０１と比べると、半導体素子の形成条件が完全には一致していないなどの理由により、発光電流特性、温度依存性などバラツキが大きい。さらに特に大きな問題となるのが波長である。各ＬＤユニット３１０のＬＤから出射されるレーザ光の波長差は、発光光量のバラツキには直接的には関係しないが、感光体の分光感度特性がすべての波長について一定でないことから、発光光量のバラツキと同じ問題を引き起こすおそれがある。
本実施例９の構成の画像形成装置では、かかる問題が発生することがない。すなわち、実施例１の場合と同じ理由により、各ＬＤユニット３１０の波長差に起因して画像上に濃度ムラがのってしまうような異常画像の発生を防止することができる。さらに、本実施例１０では、コスト的に安い１ｃｈタイプのＬＤユニットを使用することができるため、露光装置３の低コスト化を図ることができる。
【００８１】
（実施例１０）
図１４は、実施例１０に係る画像形成装置における露光装置３の概略図である。この露光装置３では、実施例１とは異なり、２組の光学ユニット３２０を感光体ドラム１の表面１ａに対して並列に配置したような構成になっている。各光学ユニット３２０はそれぞれ、ＬＤユニット３２１、コリメートレンズ３２２、シリンドリカルレンズ３２３、ポリゴンミラー３２４及びｆ−θレンズ３２５を備えている。本実施例１０のように２組の光学ユニット３２０を感光体ドラム１の表面１ａに対して並列に配置した構成では、レーザービームＬＢが感光体ドラム表面１ａ上を走査する区間を短くすることができるため、次のようなメリットがある。
▲１▼幅の広い画像に対応することができる。（いわゆる広幅機）
▲２▼レーザービームの走査区間が短いためビーム径を細くしやすい。
▲３▼画素クロックを遅くできる。
【００８２】
その反面、図１４でもわかるように、レーザービームＬＢのつなぎ目が画像中央に現れてしまうという問題がある。ＬＤ間に光量差が存在する場合、画像中央部であるために、写真画像などのように中間調（中濃度）を多く含む画像などでは、このつなぎ目で濃度差が発生してしまうという問題がある。この問題に対しても、擬似中間調処理法として周期的な構造を持たない誤差拡散法などを用いた場合には、光量差が画像上に濃淡として現れるようなことがない。そのため、上記画像中央部のつなぎ目の問題が発生しない画像形成装置を実現することができる。
【００８３】
（実施例１１）
実施例１１では、露光装置に、ＬＥＤ（発光ダイオード）を用いたＬＥＤ書きこみ光学系を採用している。このＬＥＤ書きこみ光学系は、上記実施例１等のＬＤを用いた方式に比べて、▲１▼ポリゴンミラーなどの駆動部がないことによる騒音の低減、▲２▼光学ユニット自体の大きさが小さくてすむことによる省スペース化の実現、といった長所がある。その反面、多数の発光素子が近接して配置されるため、実施例１と同じような熱的・電気的なクロストークが発生しやすいといった問題がある。また多数の発光点を駆動するため、ＬＤＥ駆動用のＩＣチップをやはり複数搭載する必要がある。このＬＥＤ駆動用のＩＣにしても特性がすべてにおいて一致させることはきわめて困難である。
本実施例１１の構成の画像形成装置では、かかる問題が発生することがない。すなわち、実施例１の場合と同じ理由により、各ＬＥＤ駆動用ＩＣの特性のばらつきに起因して画像上に濃度ムラがのってしまうような異常画像の発生を防止することができる。
【００８４】
以上、本実施形態によれば、主・副走査方向の解像度に基づいて算出される１画素の長さをそれぞれＬｐ１、Ｌｐ２とし、主・副走査方向における感光体ドラム表面上での照射光のビーム径をそれぞれＤ１、Ｄ２としたとき、条件式Ｄ１＜Ｌｐ１及びＤ２＜Ｌｐ２を満たすようにしている。これにより、上記実施例１で説明したように、電子写真方式を用いた画像形成装置における特有の問題を解決できる。すなわち、特定の周期構造を持たない誤差拡散法を使用した場合に画質上の重要な項目である「粒状性」の悪化という問題を解決することができる。これは、ビーム径を特定の範囲内に制限することによって、現像工程での問題である現像電界の周波数依存性の影響を受けづらくなるためである。この現像の空間周波数依存性とは、現像電界が書き込みのパターン（画像の空間周波数）によって変化することを意味するものであり、そして結果として、擬似中間調処理後の画像データには本来存在しないような低周波の濃度変動が現像後の画像には発生するといった問題を引き起こす。この「粒状性」の悪化は、本来均一であるべき画像領域においてざらついた印象をあたえ、特に写真などのピクトリアル画像においては画質劣化の非常に大きな原因となる。本実施形態では、擬似中間調処理法として特定な周期構造を持たない誤差拡散法を使用しても、粒状性の悪化することのない電子写真方式の画像形成装置を実現することができる。
さらに、擬似中間調処理法として特定な周期構造を持たない誤差拡散法を使用することにより、次の▲１▼及び▲２▼の長所を有する画像形成装置を実現することができる。
▲１▼モアレが発生することのなく中間調を表現することができる。
▲２▼書き込みの位置ズレなどの影響が画像上に現れることなく中間調を表現することができる。
また、本実施形態によれば、体積平均粒径が５．５μｍ以下のトナーを用いることにより、前述の数５で定義される粒状度を３．０以下にすることができる。このように粒状度を３．０以下にすることで、印刷をはじめとする電子写真方式以外のハードコピー機器と比較しても遜色なく、高画質の要件を十分満たした画像形成装置を実現することができる。
【００８５】
また、本実施形態によれば、誤差拡散法を用いた擬似中間調処理を行なっている。この誤差拡散法はＦＭスクリーンと呼ばれる擬似中間調処理法であり、ドットの密度を変化（周波数変調）させて画像の濃淡を再現する方法である。このため、周期的な構造を持たず、上記▲１▼及び▲２▼と同じ効果ををもつ。さらに、誤差拡散法は、入力画像から擬似中間調処理後画像を得る際に、各画素ごとに入力画像の値と処理後画像との差を周辺の画素に振り分けるため、平均的に見ると画像濃度の再現性が良い。
これに対して、ディザ法では入力画像と処理後画像との差が生じた場合には、この分の誤差は無視されるので、画像濃度の再現性という観点ではわずかに誤差拡散法に劣る。このほか、ブルーノイズ法なども、画像濃度の再現性という観点ではわずかに誤差拡散法に劣る。
【００８６】
また、本実施形態によれば、露光装置３が複数の発光部（ＬＤ：レーザーダイオード）を有する場合の濃度ムラを抑制できる。複数の発光部を有する場合には、各発光部での光量の差を原因とするような画像上に知覚することができるような空間周波数（５０〜１００ｌｐｉ程度）の濃度ムラが発生するといった問題がある。各発光点の光量は、先述したとおり、▲１▼電気的クロストーク、▲２▼熱的クロストーク、▲３▼発光素子の経時劣化の違い、▲４▼発光素子の温度依存性の違い、などによって光量差が生じてしまう。このような光量差によって、先述の画像上に本来存在しないような濃度ムラが発生してしまう。また、複数の画素を同時に走査して書き込むため、発光点の光量を完全に一致させた場合であっても走査の仕方が異なる（図１５参照）ため、光量差がある場合と同じような問題が発生する。図１５のケース１は、２ドットからなるパターンをレーザビームの１回の走査（走査１）で書き込む場合を示している。図１５中のケース２は、ケース１と同じ２ドットからなるパターンをレーザビームの２回の走査（走査１、走査２）で書き込む場合を示している。
このような問題に対しても、擬似中間調処理法として周期的な構造をもたない誤差拡散法を組み合わせることが有効である。周期的な構造をもたない誤差拡散法を採用することにより、上述のようなＬＤ間の光量差が発生するような場合であっても、異常画像が発生することのない画像形成装置を長期間に渡って実現することができる。また、環境変化の大きな使用条件下においても異常画像が発生することのない画像形成装置を実現することができるようになる。また、複数ＬＤを使用せざるを得ないような高速の画像形成装置において、上述のような画像形成装置を実現することができるようになる。
【００８７】
また、本実施形態では、露光装置３の光源として、複数の発光部が十数μｍ〜数十μｍの間隔で１チップ上にアレイ状に配列してある発光部材を用いることができる。このような発光部材を用いた場合は、次のような効果を奏することができる。まず、発光部同士が極めて近いため、光学ユニット内を通過する各光源の光路ほぼ同じであることある。このことは、光学ユニット内に配置される光学素子（ｆ−θレンズ、折り返しミラーなど）を上記光路が通過する位置が極めて近いことを意味する。このため、光学素子の特性が近い場所（部位）を複数の光源が使用できるようになる。従って、▲１▼感光体上でのレーザービームの露光位置が各光源について一致させやすい、▲２▼感光体上でのレーザービームのいわゆるスポット径を一致させやすい、といった長所がある。さらに、半導体レーザーダイオードの場合には、製造上の理由により、１チップ上の形成されるＬＤアレイでは、各発光部（発光源）から出射されるレーザ光の波長差が非常に小さいといった特徴がある。このようにレーザ光の波長差が小さくことにより、感光体ドラム表面上での露光位置が発光部（発光源）ごとにズレるといった現象が発生しにくくなる。
発光部がアレイ状に配列した発光部材では、上記のような長所がある反面、次に挙げるような欠点を併せ持つ。この種の発光部材では、先述したように各発光部間の距離が十数μｍ〜数十μｍの間隔であるため、各発光部の発熱によって１つの発光部の光変調の影響が隣接する発光部に光量の変動として影響する。いわゆる熱的なクロストークが大きくなる傾向がある。また、ＬＤの変調電流が隣接する発光部に電気的誘導を引き起こすような電気的なクロストークも大きい。これらの熱的なクロストークや電気的なクロストークの問題に対しても擬似中間調処理法として周期的な構造をもたない誤差拡散法を組み合わせることが有効である。周期的な構造をもたない誤差拡散法を採用することにより、出力画像上にはクロストークによる各発光部の光量差によって引き起こされる異常画像（副走査方向への画像濃淡）が発生しなくなる。このため、上述の発光部がアレイ状に配列した発光部材の長所のみが生かせるようになり、位置精度の高い画像形成装置を実現することができるようになる。
【００８８】
また、本実施形態では、感光体ドラム１の感光体として、積層型の感光体を用いることができる。感光体としては積層感光体と単層感光体とがあるが、積層型感光体の方が▲１▼高感度を有する、▲２▼光書き込み後、電荷がキャンセルされるまでの時間が短い（応答性がよい）、といった長所がある。その反面、積層型の感光体では、感光体の底面側に電荷発生層が存在するため、フォトキャリアが電荷輸送層中を長い距離（電荷輸送層膜厚に相当する１０〜３０μｍ）を移動しなければならない。この移動の間にフォトキャリア同士のクーロン反発力によってフォトキャリアが感光体の表面方向に広がってしまうといった構造上の問題がある。このようなフォトキャリアの拡散によって、感光体上に形成される静電潜像（感光体上の電荷分布）はビーム径に比べてかなり大きなものになってしまい、また電荷のコントラストとしてもブロードな形状になってしまう。ビーム径に比べての静電潜像の広がりは、擬似中間調処理法としてディザ法などのＡＭスクリーンを使用した場合にはさほど大きな問題にはならない。これは、ディザ法などでは擬似中間調処理をドット集中型で行うことが多く、このような静電潜像の広がりは網点を形成するドットのうち周辺部に位置するドットのみに影響し、内部に存在するドットには影響がないためである。しかし、誤差各拡散法などのＦＭスクリーンを使用する場合には影響が大きい。これは、誤差拡散法などでは、ドットが離散的に形成されることが多く、すべてのドットが周辺部に存在すると考えることができる。このため、静電潜像の広がりの影響がより大きいのである。静電潜像の広がりは結果として、ビーム径の大きな条件での結果と同じであり、周期的な構造をもたない擬似中間調処理方法と組みあわせた場合には粒状性が悪化するといった問題を引き起こす。
本実施形態では、主・副走査方向の解像度に基づいて算出される１画素の長さをそれぞれＬｐ１、Ｌｐ２とし、主・副走査方向における感光体ドラム表面上での照射光のビーム径をそれぞれＤ１、Ｄ２としたとき、条件式Ｄ１＜Ｌｐ１及びＤ２＜Ｌｐ２を満たすようにしている。このため、静電潜像の広がりやすい積層型の感光体と、周期的な構造をもたない擬似中間調処理方法とを組み合わせた場合においても、粒状性の悪化を抑制することが可能となる。さらに、積層型の感光体を使用することができることにより、高感度であり、高速応答の感光体を搭載した画像形成装置を実現することができ、ＬＤ出力の低減によるＬＤコストの軽減や、高速化の画像形成装置の実現といった改善が可能となる。
【００８９】
特に、上記積層型の感光体を用いる場合は、電荷輸送層（ＣＴ層）の厚さをｄとし、たとき、ｄ＜Ｄ１及びｄ＜Ｄ２の条件を満たす必要がある。この条件が必要な理由は、次のように考えることができる。感光体が積層型感光体である場合には、電荷発生層で発生したフォトキャリアが電荷輸送層を通過して感光体表面の電荷と結合する際に、静電気的なクーロン力により、電荷輸送層中においてフォトキャリアが感光体面方向に広がる現象が発生する。また、静電気的なクーロン力により、別の箇所で発生したフォトキャリアの影響を受けて感光体面方向に移動してしまう現象も発生する。これらの現象の結果として、静電潜像がひろがってしまいシャープな潜像が形成されなくなる。感光体上に形成される潜像からの観点では、大きなビーム径による書きこみと変わらなくなってしまう。また、フォトキャリアの重心位置が正規の位置から動いてしまい画像上のノイズの要因となる。
このような影響は、電荷輸送層の膜厚が大きいほど大きくなる。電荷輸送層の膜厚がビーム径よりも小さい場合（ｄ＜Ｄ１、かつｄ＜Ｄ２）には、上述の影響はあまり大きくなく、潜像のシャープさはほぼ書きこみを行うビーム径によって決定されると考えられる。しかしながら、電荷輸送層の膜厚がビーム径よりも大きな場合（ｄ＞Ｄ１、ｄ＞Ｄ２）には、書きこみのビーム径を小さくした場合においても、潜像のシャープさは電荷輸送層の膜厚によって決まってしまうため、本発明の目的とする粒状性の悪化の防止という効果は得られない。つまり、積層型感光体を使用した場合には、電荷輸送層膜厚ｄが、ｄ＜Ｄ１、かつｄ＜Ｄ２の条件を満たす場合に、粒状性の悪化の防止することができうる。
【００９０】
また、本実施形態の画像形成装置で用いるトナーを構成する樹脂に対する色材（顔料）の質量の比率は３．０〜８．０％の範囲が好ましい。この場合は、粒状性の悪化を更に確実に防止できる。
【００９１】
また、本実施形態の画像形成装置において、解像度は６００ｄｐｉ以上が好ましい。ここまで述べてきた効果については、主に画像上の中間調部分についての効果に注目したものであった。具体的には、擬似中間調処理を施した部分についての「粒状性」についてのものであった。この中間調領域の粒状性に関しては画像形成装置の解像度は４００ｄｐｉ以上であれば問題はなかった。しかしながら、文字部や細線のジャギーに関しては、解像度が４００ｄｐｉ程度では問題となる場合がある。特にトナーの体積平均粒径が小さくなった場合には、トナー像自体の凸凹がかなり円滑になりその分書き込みの解像度不足が知覚されるようになる。特に、本実施形態のようにトナーの体積平均粒径が５．５μｍ以下となったような場合では、解像度が４００ｄｐｉではジャギーが目立つようになり、画質上好ましくなかった。そこで、解像度を６００ｄｐｉに上げることによって、このようなジャギーを解決することができ高画質化を達成することができた。
【００９２】
また、本実施形態において、１画素に対する発光のデューティー比は５０％以下が好ましい。この発光のデューティー比を大きくした場合には、１画素あたりの書き込み面積が大きくなってしまう。これは、レーザーが発光している間もレーザービームは感光体ドラム表面を走査しているためである。図１６及び図１７はそれぞれデューティー比が５０％の場合と１００％の場合におけるレーザービームの走査の様子を模式的にあらわしたものである。各図中の実線で表示したものは１画素に対する照射開始時及び照射終了時のレーザ光の照射スポットである。図１６に示すように１画素に対する発光のデューティー比が５０％の場合は、発光時間中にレーザービームが感光体ドラム表面上を走査する面積が少ない。これに対し、図１７に示すように１画素に対する発光のデューティー比が１００％の場合は、感光体上を走査する面積が大きくなってしまう。つまり、上記発光のデューティー比を大きくしてしまうと、レーザービームの書き込みエネルギーが集中されずに広がってしまい、ビーム径を絞ったことの効果が得られなくなってしまう。レーザービームの書き込みエネルギーが広がってしまった場合には、上述の感光体による静電潜像の広がりと同じ理由により、周期的構造をもたない中間調処理方法と組み合わせた場合には粒状性が悪化すると考えられる。
そこで、１画素に対する発光のデューティー比を５０％以下にすることにより、露光エネルギーを集中させることができる。そして、周期的な構造をもたない擬似中間調処理法と組み合わせても、粒状性の悪化が発生しない画像形成装置を実現することができる。
【００９３】
また、本実施形態における１画素の長さＬｐ１，Ｌｐ２と照射光のビーム径Ｄ１，Ｄ２との関係（Ｄ１＜Ｌｐ１，Ｄ２＜Ｌｐ２）と誤差拡散法と５．５μｍ以下の小粒径トナーとを組み合わせる構成は、複数色のトナー像を重ね合わせた画像の出力をおこなうカラー画像形成装置において有効である。この種のカラー画像形成装置では、色重ねによる干渉を回避することが要求される。また、わずかな版ズレなどの影響が実質的に現れないようにするためにも、中間調処理方法の周期構造を、各色ごとに一致させないことが有効な対処法である。ディザ法を使用する場合には、いわゆるスクリーン角を角色ごとにずらすことがしばしば利用される。しかしながら、ディザ法を使用した場合にはモアレが発生しやすく、また、書き込み位置精度や複数ＬＤを使用する場合にはＬＤ光量差を原因とする濃度ムラが発生しやすいといった問題が発生しやすい。
これに対して、本実施形態における誤差拡散法などのように副走査方向に周期的な構造を持たない擬似中間調処理法を使用する画像形成装置では、上述のように、画像上に濃度ムラが載ることはない。また、上述の色重ねによる干渉ムラや版ズレによる色変わりについても、副走査方向に周期的な構造を持たない擬似中間調処理をおこなうため、問題が発生しない。その上、カラー画像上での粒状性の悪化も抑制することができる。
【００９４】
また、本実施形態における１画素の長さＬｐ１，Ｌｐ２と照射光のビーム径Ｄ１，Ｄ２との関係（Ｄ１＜Ｌｐ１，Ｄ２＜Ｌｐ２）と誤差拡散法と５．５μｍ以下の小粒径トナーとを組み合わせる構成は、シアン、マゼンタ、イエロー及びブラックの４色のトナー像を重ね合わせてカラー画像を形成する場合にも有効である。この場合に、擬似中間調処理としてディザ法を使用しようとするときは、４色重ねによる干渉ムラや版ズレによる色変わりを防止するために、ディザマトリクスを４枚用意し、スクリーン角をつけて組み合わせる必要ある。しかしながら、ディザ法を使用した場合にはモアレが発生しやすく、また、書き込み位置精度や複数ＬＤを使用する場合にはＬＤ光量差を原因とする濃度ムラが発生しやすいといった問題が発生しやすい。
これに対して、本実施形態における誤差拡散法などのように副走査方向に周期的な構造を持たない擬似中間調処理法を使用する画像形成装置では、問題としている画像上に濃度ムラがのるようなことはない。また、上述の色重ねによる干渉ムラや版ズレによる色変わりについても、副走査方向に周期的な構造を持たない擬似中間調処理をおこなうため、問題が発生しない。その上、４色のトナー像を重ね合わせたカラー画像上での粒状性の悪化も抑制することができる。
【００９５】
なお、本発明を適用可能なカラー画像画像形成装置の構成は、特定の構成に限定されるものではない。例えば、本発明は、タンデムタイプ（直接転写）のカラー画像形成装置、タンデムタイプ（中間転写）のカラー画像形成装置、リボルバータイプのカラー画像形成装置等に適用することができる。
【００９６】
図１８は、いわゆるタンデムタイプ（直接転写）のカラー画像形成装置の概略構成図である。このカラー画像形成装置は、張力ローラ１５に掛け回された紙搬送ベルト１６で搬送される記録シートの搬送方向に沿って、４組の画像形成部１７〜２０が配設されている。各画像形成部１７〜２０は、前述の画像形成装置（図２）と同様に、感光体ドラム１や帯電装置２等を備え、互いに異なる色（シアン、マゼンタ、イエロー、ブラック）のトナー像を形成するものである。各画像形成部１７〜２０の感光体ドラム１に形成されたトナー像は、感光体ドラム１と転写ローラ２１とが対応している転写位置で、記録シート６上に順次重ね合わせるように転写される。カラー画像が転写された記録シート６は、定着装置８でトナー画像が定着された後、機外に排出される。なお、紙搬送ベルト１６の表面に付着している残トナー等の汚れは、ベルトクリーニング装置２２でクリーニングされる。
【００９７】
図１９は、いわゆるタンデムタイプ（中間転写）のカラー画像形成装置の概略構成図である。このカラー画像形成装置は、張力ローラ１５に掛け回された中間転写体としての中間転写ベルト２３の張架方向に沿って、４組の画像形成部１７〜２０が配設されている。各画像形成部１７〜２０は、前述の画像形成装置（図２）と同様に、感光体ドラム１や帯電装置２等を備え、互いに異なる色（シアン、マゼンタ、イエロー、ブラック）のトナー像を形成するものである。各画像形成部１７〜２０の感光体ドラム１に形成されたトナー像は、感光体ドラム１と転写ローラ２１とが対応している転写位置で、中間転写ベルト２３上に順次重ね合わせるように転写される。中間転写ベルト２３上に転写されたカラー画像は、２次転写用の転写ローラ２４が対向する２次転写位置で記録シート６に一括転写される。カラー画像が転写された記録シート６は、定着装置８でトナー画像が定着された後、機外に排出される。なお、中間転写ベルト２３の表面に付着している残トナー等の汚れは、ベルトクリーニング装置２２でクリーニングされる。
【００９８】
図２０は、いわゆるリボルバータイプのカラー画像形成装置の概略構成図である。このカラー画像形成装置は、一つの感光体ドラム１上に互いに異なる色（シアン、マゼンタ、イエロー、ブラック）に対応する静電潜像が形成される。感光体ドラム１上の各色の静電潜像は、リボルバ現像装置４０が順次回転することにより、対応する色のトナーで現像される。感光体ドラム１上に形成される各色のトナー像は、転写ローラ２６が対向する転写位置で、紙搬送ドラム２５に保持されている記録シート６上に順次重ね合わせて転写される。すべての色のトナー像が転写された記録シート６は、定着装置８でトナー画像が定着された後、機外に排出される。
【００９９】
【発明の効果】
請求項１乃至１１の発明によれば、主・副走査方向における像担持体上の照射光のビーム径Ｄ１，Ｄ２をそれぞれ、主・副走査方向の解像度から算出される１画素の長さＬｐ１，Ｌｐ２よりも小さくしている。これにより、モアレが発生しにくく階調再現性にも優れた誤差拡散法を使用しても、現像電界の空間周波数依存性の影響を受けやすい低空間周波数の画像部分が発生しにくくなる。従って、低空間周波数の画像部分の発生に起因した粒状性の悪化を抑制することができる。特に、体積平均粒径が５．５μｍのトナーを用いることにより、前述の数５で定義される「粒状度」が３．０以下の良好な粒状性を達成できるという効果がある。
【図面の簡単な説明】
【図１】本発明の実施形態に係る画像形成装置における感光体ドラム表面上のレーザビームＬＢのビーム径Ｄ１，Ｄ２と１画素の長さＬｐ１，Ｌｐ２との関係を示す説明図。
【図２】同画像形成装置の概略構成図。
【図３】画像処理部を含む制御系の要部を示すブロック図。
【図４】（ａ）は露光装置における光学ユニットの平面図。
（ｂ）は同光学ユニットの正面図。
【図５】画像処理部の誤差拡散処理部における処理内容の説明図。
【図６】実施例１で用いたウェイトマトリックス（誤差拡散マトリックス）Ａの説明図。
【図７】実施例２で用いたウェイトマトリックス（誤差拡散マトリックス）Ｂの説明図。
【図８】実施例３で用いたウェイトマトリックス（誤差拡散マトリックス）Ｃの説明図。
【図９】粒状度の数値化の例を示すグラフ。
【図１０】誤差拡散処理を施した画像データの説明図。
【図１１】ビームスポット径による「中間的な電位の領域」の変化を示す説明図。
【図１２】顔料の比率を変化させたときのビームスポット径と粒状度との関係を示すグラフ。
【図１３】（ａ）は実施例９に係る露光装置における光学ユニットの平面図。
（ｂ）は同光学ユニットの正面図。
【図１４】（ａ）は実施例１０に係る露光装置における光学ユニットの平面図。
（ｂ）は同光学ユニットの正面図。
【図１５】同じ２ドットパターンを互いに異なる２種類のレーザビームの走査で書き込む場合を示した説明図。
【図１６】発光のデューティー比が５０％のときのレーザービームの走査の様子を示す説明図。
【図１７】発光のデューティー比が１００％のときのレーザービームの走査の様子を示す説明図。
【図１８】本発明を適用可能なカラー画像形成装置の概略構成図。
【図１９】他の実施形態に係るカラー画像形成装置の概略構成図。
【図２０】更に他の実施形態に係るカラー画像形成装置の概略構成図。
【符号の説明】
１ 感光体ドラム
１ａ 感光体ドラム表面
２ 帯電装置
３ 露光装置
４ 現像装置
５ 転写装置
６ 記録シート
７ クリーニング装置
８ 定着装置
１０ 画像処理部
１０１ ＭＴＦフィルタ処理部
１０２ γ処理部
１０３ 擬似中間調処理部
３０１ ＬＤ（レーザーダイオード）ユニット
ＬＢ レーザービーム[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile. More specifically, an image is formed by irradiating an image carrier with a beam-like irradiation light intensity-modulated based on image data obtained by pseudo halftone processing to form a latent image, and developing the latent image to form an image. The present invention relates to a forming apparatus.
[0002]
[Prior art]
Conventionally, as an image forming apparatus of this type, an electrophotographic image forming apparatus using an electrophotographic process including a photosensitive drum as an image carrier, a charging unit, an exposure device, and a developing device is known. I have. The photoreceptor drum of this image forming apparatus is formed by applying a photoreceptor to the surface of a conductor, and is driven to rotate in a predetermined direction. The charging unit charges the surface of the photoconductor to a desired potential. The exposure device exposes the photoconductor to form an electrostatic latent image corresponding to a desired image on the photoconductor. The developing device develops the electrostatic latent image created by the exposure device with toner to form a toner image on the photoconductor. The toner image on the photoreceptor is transferred onto a transfer body such as paper, and is heated by a fixing unit to be fixed on the transfer body.
[0003]
An exposure apparatus used in the electrophotographic image forming apparatus controls an LD (laser diode) as a light source based on image data, and modulates the intensity of laser light emitted from the LD to a photoconductor. The intensity-modulated laser light is imaged on a photoconductor via a collimator lens, an aperture, a cylindrical lens, a polygon mirror, and an f-θ lens, and is scanned on the photoconductor. Thus, an electrostatic latent image corresponding to a desired image can be formed on the photoconductor.
[0004]
Input image data read by an image input unit such as a scanner is sent to an image processing unit. The image processing unit performs various types of image processing such as filter processing, γ conversion processing, and pseudo halftone processing on the input image data, and passes the processing result to the video signal processing unit as output image data. The video signal processing unit converts the output image data into an image signal, and drives the LD at a predetermined timing.
[0005]
Here, the input image data has multi-value data of 8 to 12 bits per pixel in a pictorial image such as a photograph. In contrast, in an image forming apparatus (including an electrophotographic system) for forming an image output on a transfer body such as paper, the number of gradations that can be expressed per pixel is substantially very small. In order to solve such a problem, the image forming apparatus increases the resolution to 600 dpi, 1200 dpi, or the like, modulates the image density using a plurality of pixels in area, and generates a pseudo halftone image. indicate. Image processing performed in the step of converting the input image data into a pseudo halftone image is pseudo halftone processing.
[0006]
Pseudo halftone processing can generally be broadly divided into an AM screen and an FM screen. As a typical method of the AM screen, there is a dither method. Although a detailed description of the dither method is omitted, in the dither method, halftones are expressed by forming halftone dots arranged at a constant period and changing the size of the halftone dots. As described above, the dither method has a feature that the image after halftone processing has a fixed periodic structure.
On the other hand, a typical method of the FM screen is an error diffusion method. Although a detailed description of the error diffusion method is also omitted, in the error diffusion method, halftone expression is performed by changing the density of a toner-attached portion (a portion where writing is performed by a laser) to perform area gradation. As described above, since the density of the toner-attached portion is changed, the image does not have a fixed period, and the spectrum of the spatial frequency has a broad mountain shape. An image forming apparatus using this error diffusion method is disclosed in, for example, Japanese Patent Application Laid-Open No. H11-163,972.
[0007]
[Patent Document 1]
JP 2001-28689 A
[0008]
[Problems to be solved by the invention]
Conventionally, as a pseudo halftone processing method used in an electrophotographic image forming apparatus, a dither method belonging to an AM screen has been mainstream. This is based on the image quality items (1) graininess (roughness), (2) gradation, (3) color moiré, and (4) calculation load, and the dither method is used. This is because the image quality is excellent. On the other hand, when the error diffusion method is adopted in an electrophotographic image forming apparatus, good image quality cannot be obtained in terms of the above-mentioned image quality item (1) “graininess (roughness)”. Therefore, as a halftone processing method used in an electrophotographic image forming apparatus, an error diffusion method has not been mainstream. In particular, in an electrophotographic full-color image forming apparatus that is required to output a high-quality image, "graininess (roughness)" is a problem, and the error diffusion method has not been adopted.
[0009]
However, the use of the error diffusion method has the following advantages.
{Circle around (1)} As compared with an AM screen represented by a dither method, it is possible to output an image with excellent sharpness.
{Circle over (2)} The occurrence of abnormal images due to the lower writing position accuracy is less than that of the dither method.
{Circle around (3)} When input data having a periodic structure is input, moire does not occur.
{Circle around (4)} In the color image forming apparatus, color moiré does not occur.
Among these advantages, the advantages of (1) and (2) are particularly important. In recent years, there has been a demand for higher image quality of electrophotographic image forming apparatuses, that is, higher image quality by achieving both granularity and sharpness that are superior to printing methods such as offset printing. To improve the image quality, the advantage of (1) is important. In addition, in an electrophotographic image forming apparatus in which the size and complexity of the apparatus such as the printing method are not allowed (it is difficult to employ a high-precision mechanical drive system as compared with the printing method), the image writing position accuracy (printing One of the issues is to widen the allowable range of (position accuracy). In order to widen the allowable range of the writing position accuracy of the image, the advantage of (2) is important.
[0010]
The present invention has been made under the above background. An object of the present invention is to provide an image forming apparatus capable of achieving good granularity with a value of "granularity" of 0.30 or less even when employing an error diffusion method which is excellent in image sharpness and writing position accuracy and does not easily generate moire. It is to provide.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, the invention according to claim 1 includes an image carrier, pseudo halftone processing means for performing pseudo halftone processing on input image data to generate output image data, and the output image data. An exposure device that forms a latent image by irradiating the image carrier with a beam-like irradiation light based on a developing device that develops a latent image on the image carrier, and the pseudo intermediate processing unit includes: In an image forming apparatus that performs pseudo halftone processing by an error diffusion method, the length of one pixel calculated based on the resolution of the irradiation light in the main scanning direction is Lp1, and the length of one pixel in the sub-scanning direction of the irradiation light is Lp1. The length of one pixel calculated based on the resolution is Lp2, the beam diameter of the irradiation light on the image carrier in the main scanning direction is D1, and the sub-scanning of the irradiation light on the image carrier is D1. When the beam diameter in the direction is D2, D1 <Lp1 and D2 < Satisfy the relationship p2, volume average particle size of the toner in the developer used in the developing apparatus is characterized in that at 5.5μm or less.
According to a second aspect of the present invention, there is provided an image carrier, a pseudo halftone processing means for performing pseudo halftone processing on input image data to generate output image data, and a beam-like image based on the output image data. An exposure device for irradiating the image carrier with irradiation light to form a latent image; and a developing device for developing the latent image on the image carrier, wherein the pseudo intermediate processing unit is configured to perform pseudo intermediate processing by an error diffusion method. In the image forming apparatus in which the light source in the exposure apparatus has a plurality of light emitting units, the length of one pixel calculated based on the resolution of the irradiation light in the main scanning direction is represented by Lp1. The length of one pixel calculated based on the resolution of the irradiation light in the sub-scanning direction is Lp2, the beam diameter of the irradiation light on the image carrier in the main scanning direction is D1, The beam diameter of the irradiation light on the body in the sub-scanning direction is D When a, is characterized in that satisfies the relationship of D1 <Lp1 and D2 <Lp2, the volume average particle size of the toner in the developer used in the developing device is 5.5μm or less.
According to a third aspect of the present invention, in the image forming apparatus of the second aspect, the light source is a light emitting member in which a plurality of light emitting portions are arranged in an array on one chip.
According to a fourth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, the image carrier is a stacked type photoconductor in which a charge transport layer is formed on a charge generation layer, When the thickness of the charge transport layer is d, the relationship of d <D1 and d <D2 is satisfied.
According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, the mass ratio of the coloring material to the resin constituting the toner is 3.0 to 8.0%. It is assumed that.
According to a sixth aspect of the present invention, in the image forming apparatus of any one of the first to fifth aspects, the resolution is at least 600 dpi.
According to a seventh aspect of the present invention, in the image forming apparatus of any one of the first to sixth aspects, the duty ratio of the irradiation light to each pixel is 50% or less.
According to an eighth aspect of the present invention, in the image forming apparatus of any one of the first to seventh aspects, a pixel clock frequency which is a reciprocal of a writing time of the irradiation light to each pixel is 30 MHz. is there.
According to a ninth aspect of the present invention, in the image forming apparatus of any one of the first to eighth aspects, a color image is formed by superimposing a plurality of toner images of different colors on a transfer body. At least one color toner image of the toner image is a toner image obtained by forming a latent image with the exposure device and developing the latent image with the developing device.
According to a tenth aspect of the present invention, in the image forming apparatus of the ninth aspect, the plurality of toner images are four color toner images of cyan, magenta, yellow, and black.
According to an eleventh aspect of the present invention, in the image forming apparatus of the tenth aspect, among the four color toner images, three color toner images of cyan, magenta, and yellow form latent images by the exposure device. The latent image is a toner image obtained by developing the latent image with a developing device.
[0012]
In the image forming apparatus according to any one of the first to eleventh aspects, pseudo halftone processing is performed by an error diffusion method by a pseudo halftone processing means to generate output image data, thereby providing the following effects (1) to (3). Is obtained.
(1) An image having excellent sharpness can be output.
{Circle around (2)} A halftone can be expressed without the influence of the writing position shift appearing on the image.
{Circle around (3)} Halftone can be expressed without occurrence of moire.
Conventionally, when pseudo halftone processing is performed by an error diffusion method that can achieve such an effect, “granularity” deteriorates, and a problem has been caused in that a rough image is given in an image region that should be uniform. . Particularly in pictorial images such as photographs, this is a very large cause of image quality deterioration. As a result of intensive studies conducted by the present inventors on this image quality deterioration, it was found that microscopic development electric field changes depending on the spatial frequency of an image (latent image). In other words, in the image portion having a low spatial frequency, the size of the toner-attached area of the latent image is relatively large, and the spatial distribution of the developing electric field intensity in the direction along the surface of the image carrier becomes broad. As a result, in the image portion having a low spatial frequency, the toner adhesion area in each pixel is large, and the “granularity” is deteriorated to give a rough image. As described above, there arises a problem that a low-frequency density variation that does not originally exist in the image data after the pseudo halftone processing occurs in the developed image.
Therefore, in the image forming apparatuses of the first to eleventh aspects, the beam diameter D of the irradiation light is set smaller than the length Lp of one pixel calculated based on the resolution. As a result, even if an error diffusion method having no specific periodic structure is used as the pseudo halftone processing method, an image portion having a low spatial frequency that is easily affected by the spatial frequency dependence of the developing electric field is less likely to occur. . Therefore, it is possible to suppress the deterioration of the graininess caused by the generation of the low spatial frequency image portion.
Furthermore, as a result of experiments conducted by the inventors using toners having different volume average particle diameters under conditions such as the beam diameter and the like, when the volume average particle diameter of the toner is in a specific range, good “granularity” is obtained. Is obtained. That is, it was found that when the volume average particle diameter of the toner was 5.5 μm or less, the value of “granularity” obtained by a predetermined calculation formula could be 0.30 or less. It is considered that the condition that the value of the “granularity” is 0.30 or less is not inferior to hard copy devices such as printing other than the electrophotographic image forming apparatus, and sufficiently satisfies the requirements of high image quality. Condition. The definition of the “granularity” will be described later in detail.
The lower limit of the volume average particle diameter of the toner is preferably 3 μm or more from the viewpoints of cost reduction of toner production and prevention of toner scattering. A more preferred range is 4.5 μm or more.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment in which the present invention is applied to an electrophotographic color image forming apparatus (hereinafter, referred to as “image forming apparatus”) will be described.
FIG. 2 is a schematic configuration diagram of the image forming apparatus according to the present embodiment. This image forming apparatus includes a photosensitive drum 1 as an image carrier, a charging device 2, an exposure device 3, a developing device 4, a transfer device 5, a cleaning device 7, a fixing device 8, and the like. The charging device 2 charges the surface of the photosensitive drum 1 rotating in the direction of the arrow in the drawing to a desired potential. The exposure device 3 exposes the surface of the photoconductor drum 1 to form an electrostatic latent image corresponding to a desired image on the photoconductor drum 1. The developing device 4 develops the electrostatic latent image formed by the exposure device 3 with toner, and forms a toner image on the photosensitive drum 1. The transfer device 5 transfers the toner image on the photosensitive drum 1 onto a recording sheet 6 as a transfer body such as paper conveyed by conveyance means (not shown). The cleaning device 7 cleans toner remaining on the surface of the photosensitive drum without being transferred onto the recording sheet 6 by the transfer device 5. The recording sheet 6 to which the toner image has been transferred by the transfer device 5 is conveyed to the fixing device 8. In the fixing device 8, the toner is heated and fixed on the recording sheet 6.
Since the photoreceptor drum 1 rotates in the direction of the arrow in FIG. 2, a desired image is formed on the recording sheet 6 by repeating the steps of charging, exposure, development, transfer, and the like.
[0014]
The exposure device 3 of the present image forming apparatus performs light modulation so that a beam-like laser beam emitted from a so-called LD (laser diode) corresponds to an output image. The laser light emitted from the LD forms an image on the photosensitive drum 1 via a so-called collimator lens, aperture, cylindrical lens, polygon mirror, and f-θ lens. The polygon mirror is a rotating polygon mirror, and the laser light scans the photosensitive drum 1 by this rotation. Therefore, the surface of the photosensitive drum can be exposed to form an electrostatic latent image corresponding to a desired image.
[0015]
FIG. 3 is a block diagram illustrating a main part of a control system including the image processing unit 10. The image input unit 11 corresponds to a scanner such as a digital copier, and sends input image data obtained by reading a document to the image processing unit 10. The MTF filter processing unit 101, the γ conversion processing unit 102, and the pseudo intermediate processing unit 103 serving as a pseudo intermediate processing unit of the image processing unit 10 perform various processing such as filter processing, γ conversion processing, and pseudo halftone processing on the input image data. Perform image processing. Then, the processing result is transferred to the video signal processing unit 12 as image data for output. The video signal processing unit 12 converts the output image data into an image signal, and drives the LD of the exposure device 3 at a predetermined timing. In an image forming apparatus equipped with a plurality of LDs, the video signal processing unit 12 distributes image signals for each LD to be used.
[0016]
The toner used in the developing device 4 may be a pulverized toner or a polymerized toner depending on the manufacturing method, and each is a mixture of a resin called a binder resin and a pigment. Further, the toner may contain a wax, a charge control agent, and the like in some cases. Further, silicon oxide or titanium oxide may be externally added in some cases.
[0017]
As the photoconductor of the photoconductor drum 1, for example, an organic photoconductor can be used. This organic photoreceptor is formed by laminating an undercoat layer (UL layer), a charge generation layer (CG layer), and a charge transport layer (CT layer) in this order on an aluminum drum. Body.
[0018]
Next, more specific examples of the image forming apparatus of the present embodiment will be described together with comparative examples.
(Example 1)
The photoconductor drum 1 of the first embodiment is formed by applying a photoconductor with a thickness of 25 μm on the surface of a conductor (such as aluminum), and rotates in the direction of the arrow in FIG. This photoreceptor is a laminated OPC having a so-called charge transport layer (CT layer) of 20 μm, a charge generation layer (CG layer) of 1 μm, and an undercoat layer (UL layer) of 4 μm. The diameter of the photosensitive drum 1 is 90 mm, and the peripheral speed is 245 mm / sec.
The charging device 2 is a so-called contact roller type charging device. A charging device having a configuration in which a so-called elastic layer (thickness: 3 mm) having medium resistance is formed on a cored bar is charged with a DC voltage (−1) by a power supply. .21 kV) to uniformly charge the photoconductor (-550 V).
The exposure device 3 forms an electrostatic latent image by irradiating the surface of the photoconductor uniformly charged by the charging device 2 with light corresponding to a target image. The light source of the exposure apparatus is an LD (laser diode), which scans the photosensitive member with a polygon mirror while irradiating the photosensitive member with a laser beam. The so-called beam diameter is 40 μm (= D1) in the main scanning direction and 40 μm (= D2) in the sub-scanning direction.
The developing device 4 is a so-called two-component developing device in which a developer obtained by mixing a toner (volume average particle diameter of 5.5 μm) and a carrier (particle diameter of 35 μm) at a toner concentration of 6.5% is stored in a developing container. I have. In the developing device 4, the developer is conveyed to the photoconductor-developing sleeve facing portion by the developing sleeve 4a. The distance between the photosensitive member and the developing sleeve, that is, the so-called developing gap is 0.3 mm. A DC voltage (-400 V) is applied to the developing sleeve 4a from a power source (not shown), and reversal development is performed in which toner adheres to the photoconductor in accordance with the electrostatic latent image on the photoconductor. The peripheral speed of the developing sleeve 4a is 490 mm / sec. The ratio of the peripheral speed of the developing sleeve 4a to the peripheral speed of the photosensitive drum 1 (peripheral speed ratio) is 2.0.
The transfer device 5 transfers the toner image developed by the developing device 4 onto the recording sheet 6 conveyed from a paper feeding unit (not shown). The transfer device 5 according to the first embodiment includes a transfer belt 5b wound around two rollers 5a and a power supply (not shown), and a voltage is applied to the transfer belt 5b from the power supply. The applied voltage is constant current control and is 30 μA.
The cleaning device 7 includes a blade 7a formed of an elastic body and the like, and cleans a residual toner image on the photosensitive drum 1, that is, a so-called transfer residual toner.
The toner image transferred onto the recording sheet 6 such as paper by the transfer device 5 is conveyed to a fixing device 8 and is heated and pressed by the fixing device 8 so that the toner image is fixed on the recording sheet 6 to form an image. The image is discharged out of the apparatus and becomes an output image.
Also in the first embodiment, a desired image can be formed on a recording sheet by repeating the above-described steps of charging, exposing, developing, transferring, cleaning, and fixing.
[0019]
FIGS. 4A and 4B are a plan view and a front view, respectively, of the exposure apparatus (optical writing unit) 3 according to the first embodiment. In the first embodiment, an LD unit 301 including a two-channel LD array in which two light emitting points are formed on one chip is mounted as a light source. Laser light from each LD of the LD unit 301 is applied to a polygon mirror 306 via a so-called collimating lens 302, an ND filter 303, an aperture 304, and a cylindrical lens 305. The polygon mirror 306 of the first embodiment is of a five-sided type, and rotates at a rotation speed of 34724 rpm. The laser light reflected by the polygon mirror 306 forms an image on the photosensitive drum 1 via the f-θ lens 307 and the folding mirror 308. In the first embodiment, the beam diameter of the laser beam LB on the photosensitive drum 1 is adjusted to be 40 μm (= Lp1: main scanning direction) × 40 μm (= Lp2: sub-scanning direction). The f-θ lens 307 is a plastic lens formed by molding plastic, and the lens shape is designed by an AC surface (free-form surface). As a result, an extremely narrow beam diameter of 40 μm (= Lp1: main scanning direction) × 40 μm (= Lp2: sub-scanning direction) is realized. The adjustment of the beam diameter was performed while measuring the beam diameter using a BeamScan device manufactured by PHOTON. In this device, 1 / e of the peak value ² The diameter at the value of is described as the beam diameter.
[0020]
The laser beam LB scans the photosensitive drum 1 by rotating the polygon mirror 306. The resolution of the image forming apparatus of the first embodiment is 600 dpi, and the size of one pixel is 42.3 μm (main scanning direction) × 42.3 μm (sub scanning direction). In the first embodiment, the photosensitive drum 1 is irradiated with the laser beam LB while moving per pixel for 25.5 nsec. At this time, the frequency of the pixel clock, which is the reciprocal of the optical writing time for each pixel, is 39.2 MHz, which means that the LD is optically modulated at the frequency of 39.2 MHz.
[0021]
In the first embodiment, as described above, the laser light scans the photosensitive drum by the rotation of the polygon mirror 306. When the laser light is located in the non-image area, the laser light is illustrated in FIG. The laser light is incident on the synchronization detection plate 309. The synchronization detection plate 309 has a mechanism that generates a reference signal by the incidence of the laser beam LB, and resets a timing of an image writing position, that is, a clock signal that forms a so-called pixel clock, based on the reference signal. It has become. Thereby, a laser beam subjected to light modulation can be incident on a predetermined position on the photosensitive drum 1.
[0022]
In the first embodiment, the recording start position (vertical registration) in the sub-scanning direction is determined as follows. First, the paper position is detected by a paper detection sensor disposed on the paper transport path. Then, based on the recording start signal generated by the recording start signal generating means generated based on this detection signal, the drive of the LD starts, and the optical writing to the photosensitive drum 1 starts. In the first embodiment, the recording start position is detected by the above-described paper detection sensor. However, the recording start position may be detected by another method. For example, a rotary encoder built in the photoconductor driving unit, a linear encoder that reads a pattern printed on the surface of the photoconductor drum, or the like may be used. Further, in a color image forming apparatus in which four plates of C, M, Y, and K are superimposed on an intermediate transfer member such as an intermediate transfer belt, a method of detecting the recording start position by employing the encoder as the intermediate transfer belt may be used. . Alternatively, a method may be used in which a pattern for detecting the position is printed in advance on the plate transferred onto the intermediate transfer belt, and the print start position is detected by reading the print pattern.
[0023]
In the first embodiment, the pulse width of the LD is changed in 16 steps so as to perform so-called 4-bit writing in which 16 gradations can be expressed per pixel. I'm doing it. However, this does not mean that the pulse width modulation of the LD is 4 bits. In the first embodiment, the pulse width of the LD light emission has a function of adjusting the difference in the integrated light amount between the LDs in addition to forming the above-described gradation difference, so that the pulse width modulation can be performed with 8 bits. I have.
[0024]
Next, the image processing unit 10 according to the first embodiment will be described. Since the image forming apparatus of the first embodiment is a laser printer type, it is assumed that input image data is a multi-valued (8-bit) image from a personal computer or the like. In the case of a digital copying machine or the like, it is assumed that a scanner for reading a document is added and input data is sent from this scanner unit.
The input image data is subjected to emphasis processing in the MTF filter processing unit 101 in the image processing unit 10 shown in FIG. Next, pseudo error processing is performed by the error diffusion processing unit 103, and is delivered to the image output side (LD drive side) as output image data.
The MTF filter processing unit 101 performs a product-sum calculation using each pixel data in a fixed-size matrix and a predetermined filter coefficient to emphasize edges in the image. Since the MTF filter processing is the same as that of the conventional technique, detailed description will be omitted.
The γ correction processing unit 102 performs a conversion process for determining the density of the output image. At this time, the density characteristics are set by setting one type selected from a plurality of density characteristics prepared in advance to the γ conversion process via the system bus 13. It is done by doing. Since the γ-conversion correction processing unit 102 is the same as the conventional technology, a detailed description is omitted.
[0025]
Next, the error diffusion processing unit 103 will be described with reference to FIG. The error diffusion processing unit 103 includes an addition unit 103a, a quantization unit 103b, an error calculation unit 103c, an error storage unit 103d, and a correction value calculation unit 103e.
FIG. 6 is a diagram showing a weight matrix used in the calculation in the correction value calculation unit 103e shown in FIG. Here, the numerical value shown in FIG. 6 means the weight of the error in the corresponding pixel, and * represents the pixel of interest. Note that various configurations are conceivable for the weight matrix, and the configuration is not limited to the configuration (weight matrix A) shown in FIG. The weight matrix is stored in advance in a storage unit included in the correction value calculation unit 103e.
First, the correction value calculation unit 103e obtains the error e of the quantized pixel from the error storage unit 103d. _{x + j, y + j} And a weight matrix corresponding to this error is read from the built-in storage unit. Then, as shown below, this error e _{x + j, y + j} And the corresponding weight matrix are integrated for each pixel, the product for each pixel is added, and the sum is divided by the sum of the weights of the weight matrix to obtain a correction value E. _{x, y} Is calculated.
Here, the correction value E _{x, y} Can be expressed by the following equation when expanded using the weight matrix shown in FIG. 6 as an example.
(Equation 1)

[0026]
Then, the input image data d _{x, y} And correction value E _{x, y} Are added by the adder 103a, and the corrected pixel data D _{x, y} Is output. Next, the correction pixel data D _{x, y} Are quantized. More specifically, the quantization unit 103b is 2 in the first embodiment, and the output image data takes a value of 0 or 1.
Then, the error calculator 103c calculates the corrected pixel data D _{x, y} And output image data O _{x, y} And an error e in the pixel of interest according to a predetermined value B (255 in the first embodiment because the quantization number is 2). _{x, y} Is calculated by the following equation.
(Equation 2)

[0027]
In the first embodiment, the quantization number is 2, but another value may be used at all. In the ordinary multi-valued error minimization method, O _{x, y} The value of × B is between 0 and the maximum value of the input image data (for example, 255). _{x, y} Is a value equally divided according to the quantization number, but may be an arbitrary value. In that case, the output image image data O _{x, y} For example, a look-up table or the like in which a value according to is predetermined is used.
[0028]
The result of the processing performed by the image processing unit 10 as described above is sent to the video signal processing unit 12, which is the next step, as output image data. The video signal processing unit 12 receives the above-described output image data, stores data for the number of light emitting units (2LD in the first embodiment) on a line memory, and matches the rotation of the polygon mirror 306 (so-called synchronization). Signal), and output to the LD drive driver section as a light modulation signal. The LD drive driver unit drives the two LDs of the LD unit 301 at the same time, thereby forming a desired latent image on the surface of the photosensitive drum.
[0029]
In the first embodiment, the pseudo halftone processing method is the error diffusion method as described above. The details of the error diffusion method are as described above. In this error diffusion method, the output image data after the pseudo halftone processing does not have a periodic structure. The error diffusion method is one of pseudo halftone processing methods generally called an FM screen, and reproduces the gradation of an image by changing the dot density (spatial frequency of the image). For this reason, it has a feature that it does not have a fixed periodic structure.
[0030]
Next, the toner used in the image forming apparatus according to the first exemplary embodiment will be described. The toner used in Example 1 is a so-called polymerized toner manufactured by the following method.
[0031]
~ Synthesis of resin fine particle emulsion ~
683 parts of ion-exchanged water, 11 parts of sodium salt of methacrylic acid ethylene oxide adduct sulfate (Eleminor RS-30, manufactured by Sanyo Chemical Industries), 83 parts of styrene, 83 parts of methacrylic acid were placed in a reaction vessel equipped with a stirring rod and a thermometer. Parts, butyl acrylate and 110 parts of ammonium persulfate were charged and stirred at 400 rpm for 15 minutes to obtain a white emulsion. The mixture was heated to 80 ° C. in the system and reacted for 5 hours. Further, 30 parts of a 1% aqueous solution of ammonium persulfate is added dropwise, and the mixture is aged at 80 ° C. for 7 hours to form a vinyl resin (sodium salt of styrene-methacrylic acid-butyl acrylate-ethyl methacrylate oxide adduct sulfate). Thus, an aqueous dispersion [Polymer dispersion 1] was obtained. The volume average particle diameter of [fine particle dispersion 1] measured with LA-920 was 0.09 μm. A part of [fine particle dispersion 1] was dried to isolate a resin component. The Tg of the resin component was 58 ° C.
[0032]
~ Adjustment of aqueous phase ~
1000 parts of ion-exchanged water, 83 parts of [particulate dispersion 1], 37 parts of a 48.5% aqueous solution of sodium dodecyldiphenyletherdisulfonic acid (Eleminol MON-7, manufactured by Sanyo Chemical Industries), and 90 parts of ethyl acetate are mixed and stirred. Then, a milky liquid was obtained. This is designated as [aqueous phase 1].
[0033]
~ Synthesis of low molecular polyester ~
In a reaction vessel equipped with a cooling pipe, a stirrer, and a nitrogen inlet pipe, 229 parts of bisphenol A ethylene oxide 2 mol adduct, 529 parts of bisphenol A propylene oxide 3 mol adduct, 208 parts of terephthalic acid, 46 parts of adipic acid and dibutyl After adding 2 parts of tin oxide and reacting at 230 ° C. for 8 hours under normal pressure, and further reacting at 5:00 under reduced pressure of 10 to 15 mmHg, 44 parts of trimellitic anhydride were added to the reaction vessel, and the reaction was carried out at 180 ° C. and normal pressure. After reacting for 2 hours, [low-molecular polyester 1] was obtained. [Low-molecular polyester 1] had a number average molecular weight of 2500, a weight average molecular weight of 6,700, and a Tg of 43 ° C.
[0034]
~ Synthesis of prepolymer having isocyanate group ~
In a reaction vessel equipped with a cooling pipe, a stirrer, and a nitrogen introduction pipe, 682 parts of a 2 mol adduct of bisphenol A ethylene oxide, 81 parts of a 2 mol adduct of bisphenol A propylene oxide, 283 parts of terephthalic acid, and 22 parts of trimellitic anhydride Then, 2 parts of dibutyltin oxide was added, reacted at 230 ° C. under normal pressure for 8 hours, and further reacted under reduced pressure of 10 to 15 mmHg for 5 hours to obtain [intermediate polyester 1]. [Intermediate polyester 1] had a number average molecular weight of 2,100, a weight average molecular weight of 9,500, a Tg of 55 ° C., an acid value of 0.5, and a hydroxyl value of 51.
Next, 410 parts of [intermediate polyester 1], 89 parts of isophorone diisocyanate, and 500 parts of ethyl acetate were placed in a reaction vessel equipped with a cooling pipe, a stirrer, and a nitrogen introduction pipe, and reacted at 100 ° C. for 5 hours. Polymer 1] was obtained. The free isocyanate weight% of [Prepolymer 1] was 1.53%.
Met.
[0035]
~ Synthesis of Ketimine ~
170 parts of isophoronediamine and 75 parts of methyl ethyl ketone were charged into a reaction vessel equipped with a stirring rod and a thermometer, and reacted at 50 ° C. for 5 hours to obtain [ketimine compound 1]. The amine value of [ketimine compound 1] was 418.
[0036]
~ Adjustment of pigment master batch ~
1200 parts of water, 540 parts of carbon black (Printex 60, manufactured by Dexa) and 1200 parts of [low molecular weight polyester 1] are added and mixed with a Henschel mixer (manufactured by Mitsui Mining), and the mixture is heated at 130 ° C. using two rolls. After kneading for 45 minutes, it was rolled and cooled, and pulverized to 1 mmφ or less with a pulperizer to obtain [Master batch 1].
[0037]
~ Creation of oil phase ~
378 parts of [low-molecular polyester 1], 110 parts of synthetic ester wax, 22 parts of CCA (salicylic acid metal complex E-84: Orient Chemical Industry), and 947 parts of ethyl acetate are charged into a container equipped with a stirring rod and a thermometer, and stirred. The temperature was raised to 80 ° C., kept at 80 ° C. for 5 hours, and cooled to 30 ° C. in one hour. Next, 500 parts of [Masterbatch 1] and 500 parts of ethyl acetate were charged into the container and mixed for 1 hour to obtain [raw material solution 1].
1324 parts of this [raw material solution 1] was transferred to a container, and a bead mill (Ultra Visco Mill, manufactured by IMEX Co., Ltd.) was used to feed 1 kg / hr, a disk peripheral speed of 6 m / sec, and 0.5 mm zirconia beads in 80 volumes. %, Carbon black and WAX were dispersed under the conditions of 3 passes. Next, 1324 parts of a 65% ethyl acetate solution of [low-molecular polyester 1] was added, and the mixture was passed once with a bead mill under the above conditions to obtain [pigment / WAX dispersion liquid 1]. The solid content concentration (130 ° C., 30 minutes) of [Pigment / WAX dispersion liquid 1] was 50%.
[0038]
~ Emulsification ⇒ Desolvation ~
[Pigment / WAX Dispersion 1] 650 parts, [Prepolymer 1] 140 parts, [Ketimine Compound 1] 6.0 parts are put in a container, and the mixture is put in a TK homomixer (manufactured by Tokushu Kika) at 5,000 rpm. After mixing for 1 minute, 1200 parts of [aqueous phase 1] was added to the vessel, and mixed with a TK homomixer at 13,000 rpm for 20 minutes to obtain [emulsified slurry 1].
[Emulsified slurry 1] was charged into a container equipped with a stirrer and a thermometer, and after removing the solvent at 30 ° C. for 8 hours, aging was performed at 40 ° C. for 8 hours to obtain [dispersed slurry 1].
[0039]
~ Washing⇒Drying ~
[Emulsified Slurry 1] After 100 parts of filtered under reduced pressure,
{Circle around (1)} 100 parts of ion-exchanged water were added to the filter cake, mixed with a TK homomixer (at 12,000 rpm for 10 minutes), and then filtered.
{Circle around (2)} A 10% aqueous solution of sodium hydroxide (1OO parts) was added to the filter cake of (1), mixed with a TK homomixer (at 12,000 rpm for 30 minutes), and then filtered under reduced pressure.
(3): 100 parts of 10% hydrochloric acid was added to the filter cake of (2), mixed with a TK homomixer (at 12,000 rpm for 10 minutes), and then filtered.
(4): 300 parts of ion-exchanged water was added to the filter cake of (3), mixed with a TK homomixer (at 12,000 rpm for 10 minutes), and then filtered twice to obtain [Filter cake 1]. Was.
The [filter cake 1] was dried at 45 ° C. for 48 hours with a circulating drier and sieved with a mesh having a mesh size of 75 μm, and 100 parts of the toner particles were treated with hydrophobic silica (hexamethyldisilazane surface-treated product, specific surface area). : 200m ² / G) 0.5 part and hydrophobized rutile-type titanium oxide (isobutyltrimethoxysilane surface-treated product, average primary particle diameter: 0.02 μm) 0.5 part were mixed with a Henschel mixer to obtain toner A. Was. This toner had a volume average particle size of 5.43 μm, a Tg of 46 ° C., and a THF-insoluble content of the resin component of 12%.
[0040]
The particle size of the toner produced by the above method was measured with a particle size analyzer “Coulter Counter TAII” manufactured by Coulter Electronics Co., Ltd. at an aperture diameter of 100 μm. The volume average particle size and the number average particle size of the toner were determined by the above particle size measuring device. The volume average particle size of the toner prepared in Example 1 was 5.5 μm. The mass ratio of the pigment to the resin in the toner was 6.0%.
[0041]
Although the toner production method of Example 1 is as described above, the present invention can be applied to a case where a toner produced by another method is used. For example, a toner produced by a dispersion polymerization method or a pulverization method in addition to the above method may be used.
[0042]
Next, the photosensitive drum 1 used in the image forming apparatus of the first embodiment will be described.
The photosensitive drum 1 of Example 1 was coated with an undercoat layer coating solution, a charge generation layer coating solution, and a charge transport layer coating solution having the following compositions on an aluminum cylinder having a diameter of 90 mm by dip coating and then dried. It was created. By this method, a 3.5 μm-thick undercoat layer (UL layer), a 0.2 μm charge generation layer (CG layer), and a 20 μm charge transport layer (CT layer) are formed. The film thickness was measured with a film thickness meter manufactured by Fisher Scope.
[0043]
(Undercoat layer coating liquid)
・ Titanium dioxide powder: 400 parts
・ Melamine resin: 65 parts
・ Alkyd resin: 120 parts
・ 2-butanone: 400 parts
(Coating solution for charge generation layer)
Chlorogallium phthalocyanine pigment: 2 parts
Polyvinyl butyral (Eslec BM-1: manufactured by Sekisui Chemical): 1.0 part
Cyclohexanone: 30 parts
Methyl ethyl ketone: 70 parts
(Charge transport layer coating solution)
・ Polycarbonate (Panlite C-1400, manufactured by Teijin Chemicals): 6 parts
A charge transport material having the following structural formula: 4 parts
(Carrier mobility: 1 × 10 ^-5 cm ² ・ V ^-1 ・ Sec ^-1 )
・ Tetrahydrofuran: 50 parts
Embedded image

[0044]
The specifications of the photosensitive drum 1 of the first embodiment are as described above, but are not limited thereto. A photoconductor drum using another material may be used, or a single-layer photoconductor may be used instead of the stacked photoconductor of the first embodiment.
[0045]
Next, evaluation of an output image will be described. Using 17 patches determined in advance among the 256 patches (all gradations), these 17 patches (15 mm square) were read by a scanner, and the granularity was calculated and derived by the method described below. The “granularity” is defined as a “subjective evaluation value indicating how rough an image that should be uniform” is. The amount that objectively expresses the granularity, which is the subjective evaluation value, is an evaluation scale of the granularity and is “granularity”. RMS granularity is standardized as this granularity, and is standardized by ANSI PH-2.40-1985 and is represented by the following equation. In the equation, Di is a density distribution, and D is an average density (D = (1 / N) · ΣDi).
[Equation 3]

[0046]
In addition, the granularity using the Wiener Spectrum, which is the power spectrum of the density fluctuation of the image represented by the following equation, is defined. Xerox's Dooley and Shaw applied Wiener Spectrum to cascade with visual spatial frequency characteristics (Visual Transfer Function: VTF), and then integrated the value into a granularity (GS) (for details, see RP Dooley, Rshaw: Noise Perception in Electrophotography, J. Appl. Photogr. Eng., 5, 4 (1979), pp 190-196).
(Equation 4)

[0047]
The “granularity” in the present embodiment further defines the granularity based on the Dooley and Shaw, and is defined by the following equation.
(Equation 5)

[0048]
In this case, the lightness L * is used instead of the image density D. The latter is characterized in that the linearity of the color space is excellent and the adaptability to a color image is also excellent. The granularity described below is the granularity defined by the equation shown in Expression 5.
By definition, granularity represents the noise characteristics of an image. By measuring the granularity of the output image by the above-described method, the noise characteristics (roughness) of the image can be quantified. As can be seen from the definition, the numerical value of the granularity is small when the roughness is good, and increases as the roughness becomes poor. In this embodiment, after reading the output image with a scanner (FT-S5000: manufactured by Dainippon Screen Co., Ltd.), the granularity was calculated based on the calculation formula shown in Expression 5 above.
[0049]
FIG. 9 is a graph showing an example of digitization of granularity. The horizontal axis represents brightness and the vertical axis represents granularity. The granularity is given for each lightness, and the granularity is quantified for each of 17 levels (17-stage patches). Five patches whose brightness values (horizontal axis) were closest to 80, 70, 60, 50, and 40 were selected from these 17 patches. Then, a value calculated by arithmetically averaging the granularity values of the five patches is defined as the granularity under the image output condition. Hereinafter, the granularity in the present embodiment is the granularity calculated by the arithmetic mean.
[0050]
Further, the image output experiment was performed using an experimental device having the configuration described in the first embodiment (an experimental device modified based on Ricoh's IMAGIO Color 5100). As a result of measuring the image output and the granularity, the value of the granularity in the image forming apparatus having the configuration of the first embodiment was 0.20. If the value of the granularity is 0.30 or less, it is considered to be sufficiently comparable to a hard copy device other than an electrophotographic system such as printing, and sufficiently satisfies the requirements of high image quality. .30 or less was considered to be a restriction on image quality.
As described above, in the image forming apparatus according to the first embodiment, even when the error diffusion method is used as the pseudo halftone processing method, the granularity of the output image is good as described above.
[0051]
(Examples 2 and 3)
In the second and third embodiments, in the same image forming apparatus as in the first embodiment, only the weight matrix used in the error diffusion process is changed as follows. The weight matrices used in Example 2 and Example 3 are shown in FIGS. 7 and 8, respectively. The resolution in each of the first, second and third embodiments is 600 dpi (main scanning direction) × 600 dpi (sub scanning direction).
[0052]
Embodiment 1: Error diffusion A (weight matrix A in FIG. 6)
Embodiment 2: Error diffusion B (weight matrix B in FIG. 7)
Embodiment 3: Error diffusion C (weight matrix C in FIG. 8)
[0053]
Table 1 shows the evaluation results of the granularity in Examples 1, 2, and 3. With any of the weight matrices, an image forming apparatus having good graininess can be realized.
[Table 1]

[0054]
(Comparative Examples 1, 2, 3)
In Comparative Examples 1, 2 and 3, in the same image forming apparatus as in Example 1, only the beam diameters D1 and D2 on the photosensitive drum 1 were changed as follows. The beam diameter was changed by changing the aperture, and the adjusted beam diameter was measured and confirmed by beam scanning.
[0055]
Comparative Example 1: 50 μm (main scanning direction) × 50 μm (sub scanning direction)
Comparative Example 2: 60 μm (main scanning direction) × 60 μm (sub scanning direction)
Comparative Example 3: 70 μm (main scanning direction) × 70 μm (sub scanning direction)
[0056]
Table 2 shows the evaluation results of the granularity in Comparative Examples 1, 2, and 3. In all the comparative examples, the granularity was 0.3 or more.
[Table 2]

[0057]
Each of Examples 1 to 3 and Comparative Examples 1 and 2 is a binary error diffusion with a resolution of 600 dpi, and the lengths Lp1 and Lp2 of one pixel in the main scanning direction and the sub-scanning direction based on the resolution are 42.3 μm. is there. Here, when the beam diameters of the laser beam LB on the surface of the photosensitive drum in the main scanning direction and the sub-scanning direction are D1 and D2, respectively, as shown in FIG. Satisfying is a condition that does not deteriorate the granularity. From the results in Table 2, the mechanism that can prevent the deterioration of granularity by satisfying the above conditions will be described later.
(Equation 6)
D1 <Lp1
D2 <Lp2
[0058]
Next, a result of outputting an image with the resolution of the image forming apparatus set to 1200 dpi will be described. When the resolution is changed from 600 dpi to 1200 dpi, the length of one pixel in the main scanning direction and the sub-scanning direction is Lp1 = Lp2 = 21.2 μm. In order to change the resolution to 1200 dpi and output an image, in the first embodiment, the frequency of the pixel clock is 78.3 MHz, the number of rotations of the polygon mirror 306 is 34724 rpm, and the number of LDs is doubled. can do. Since a total of four LDs are required, a four-channel LD array formed by arranging four LDs on one chip was used due to restrictions on the layout of the light source.
[0059]
(Example 4)
In Example 4, the beam diameter on the photosensitive drum 1 was set to 20 μm (D1: main scanning direction) × 20 μm (D2: sub-scanning direction) by replacing the aperture. The resolution was set to 1200 dpi by the above change.
[0060]
(Comparative Example 4)
In Comparative Example 4, the beam diameter on the photosensitive drum 1 was set to 30 μm (D1: main scanning direction) × 30 μm (D2: sub-scanning direction) by replacing the aperture. The resolution was set to 1200 dpi by the above change.
[0061]
(Comparative Example 5)
In Comparative Example 5, the beam diameter on the photosensitive drum 1 was set to 40 μm (D1: main scanning direction) × 40 μm (D2: sub-scanning direction) by replacing the aperture. The resolution was set to 1200 dpi by the above change.
[0062]
Table 3 shows the evaluation results of the granularity in Example 4 and Comparative Examples 4 and 5 in which the resolution was 1200 dpi. As can be seen from Table 3, when the beam diameter becomes large (specifically, when it becomes 30 (main) × 30 (sub) μm or more), the value of the granularity is remarkably deteriorated. At 1200 dpi, as in the case of 600 dpi, satisfying the relationship of the conditional expression represented by the above-described Expression 6 is one of the conditions that do not deteriorate the granularity.
[Table 3]

[0063]
Next, the lengths per pixel based on the resolution in the main scanning direction and the sub-scanning direction are Lp1 and Lp2, respectively, and the beam diameter of the laser beam LB on the surface of the photosensitive drum in the main scanning direction and the sub-scanning direction is D1. The reason why the granularity does not deteriorate even when the error diffusion method is used as the pseudo halftone processing method when the relations D1 <Lp1 and D2 <Lp2 are satisfied when D2 and D2 are satisfied is described. Although the reason for the details is not speculative, it is an empirical explanation based on the experimental results of the inventors.
[0064]
In the error diffusion method, the image data after the pseudo halftone processing does not have a periodic structure as described above. FIG. 10 shows image data subjected to error diffusion processing (an image is a part of a patch corresponding to 80/255 and 10 × 10 pixels are extracted). In the pseudo halftone processing by the error diffusion method, as shown in FIG. Since the arrangement of the write pixels is aperiodic, the positional relationship between the adjacent write pixels is also different. There are cases where there are no written pixels around and cases where many written pixels are adjacent.
[0065]
Writing is actually performed by a laser beam corresponding to the image data as shown in FIG. 10, but the shape of the formed electrostatic latent image changes depending on the beam spot diameter of the laser beam. In the case where the beam spot diameter is larger than one pixel of the image data (D1> Lp1, D2> Lp2), exposure is performed simultaneously on pixels where writing is not performed. The potential at such a portion is an intermediate potential between the unexposed portion and the exposed portion. On the other hand, when the beam spot diameter is smaller than one pixel of the image data (D1> Lp1, D2> Lp2), the latent image is actually diffused in the plane direction by the charge transport layer of the photoconductor, and thus the There is a portion that becomes an intermediate potential as shown in FIG. The region having the intermediate potential is smaller in each stage than when the beam spot diameter is larger than one pixel of the image data as shown in FIG.
[0066]
Further, in an electrophotographic image forming apparatus, since it is inevitable that the image forming apparatus has a frequency dependency in a developing process, a toner adhesion amount differs for each frequency of a latent image. As a result of the spatial frequency dependence in the developing process, the amount of toner adhering to the pixel of interest changes depending on the surrounding written pixels. The emphasis or the dulling of the toner adhesion amount is determined by the written pixel existing around the target pixel. The influence of such frequency dependency cannot be ignored as the region having an intermediate potential between the unexposed portion and the exposed portion increases. That is, whether or not the toner adheres to the portion having the intermediate potential is determined by the surrounding conditions. In the error diffusion method, since the surrounding writing situation is not constant as shown in FIG. 10, depending on whether or not the toner adheres to such an intermediate potential portion, when the macroscopic observation is made, the low frequency is low. It appears that a density variation has occurred, and it is expected that the image will have poor graininess.
[0067]
On the other hand, when the beam spot diameter is smaller than one pixel of the image data (main scanning direction: D1 <Lp1, sub-scanning direction: D2 <Lp2), there are few portions that become intermediate potentials. This does not result in an image having poor graininess.
[0068]
(Comparative Examples 6, 7, 8)
Next, the result of a comparative experiment between the image forming apparatuses of Examples 1 to 4 and the image forming apparatus using the dither method will be described. The pseudo halftone processing used in the image forming apparatuses of Comparative Examples 6 to 9 is as follows.
Comparative Example 6: 280 lpi dither
(45 ° each screen, periodic structure in the sub-scanning direction: available)
Comparative Example 7: 240 lpi dither
(Screen angle 53 °, periodic structure in the sub-scanning direction: yes)
Comparative Example 8: 212 lpi dither
(45 ° each screen, periodic structure in the sub-scanning direction: available)
Comparative Example 9: 170 lpi dither
(45 ° each screen, periodic structure in the sub-scanning direction: available)
[0069]
The image forming apparatuses of Examples 1 to 4 using the error diffusion method as the pseudo halftone processing method have sufficiently good granularity and have the following features (1) to (3).
That is, (1) it is possible to realize an image having excellent sharpness as compared with the dither method. (2) The occurrence of abnormal images due to low position accuracy is small. In the experimental apparatus described in the first embodiment, two LDs are mounted. However, a deviation occurs in a so-called magnification error due to a wavelength difference between the two LDs, so that a writing position shift of up to about 30 μm occurs in the main scanning direction. . However, in the configuration of the first embodiment, such an influence of the writing position shift did not appear on the output image. In contrast, when the dither method was used, an abnormal image on a line extending slightly in the main scanning direction was observed. Further, (3) no moiré occurred even when input data having a periodic structure was input.
[0070]
On the other hand, in Comparative Examples 6 to 8, as shown in Table 4, no condition was found that achieves both good graininess and all of the above (1) to (3). The image quality evaluation in Table 4 is based on visual evaluation.
[Table 4]

[0071]
(Examples 5 and 6, Comparative Examples 10 and 11)
Next, an image output result when the specification of the toner is changed in the image forming apparatus of the first embodiment will be described. The volume average particle diameter of the toner is measured by a Coulter counter as described above. The toners used in Example 5 and Comparative Examples 10 and 11 are as follows.
Comparative Example 10: Toner produced by a polymerization method with substantially the same formulation as in Example 1 (volume average particle size: 6.9 μm)
Example 5: Toner prepared by a pulverization method (volume average particle size: 5.5 μm)
Comparative Example 11: Toner produced by a pulverization method (volume average particle size: 6.5 μm)
[0072]
Table 5 shows the evaluation results of the granularity in Example 5 and Comparative Examples 10 and 11. As is clear from Table 5, in the case of the toner produced by the pulverization method having a volume average particle diameter of 5.5 μm, the value of the granularity was 0.20, and good granularity was obtained. On the other hand, when the volume average particle diameter of the toner was large (6.5 μm, 6.9 μm), it was not possible to improve the granularity regardless of the type of toner preparation method.
[Table 5]

[0073]
Further, as Example 6, in the same image forming apparatus as in Example 1, an image was output using a toner produced by a polymerization method having an average volume particle size of 4.5 μm, and the granularity was evaluated. As a result, as shown in Table 6, the value of the granularity was 0.19, and good granularity was obtained.
[Table 6]

[0074]
From the results of Examples 5 and 6 and Comparative Examples 10 and 11, and the results of Example 1 described above, it can be seen that when the volume average particle diameter of the toner is at least 5.5 μm or less, the granularity is improved. Understand. In addition, it can be seen that the granularity does not depend on the toner production method (polymerization method, pulverization method).
[0075]
(Example 7)
Next, an image output result in the seventh embodiment in which the mass ratio of the pigment (color material) to the resin in the toner (hereinafter, referred to as “pigment ratio”) in the image forming apparatus of the first embodiment is changed will be described. The evaluation item of the image is the above-described granularity, and the pseudo halftone process is a binary error diffusion process with a resolution of 600 dpi using the weight matrix used in the first embodiment.
[0076]
FIG. 12 shows the results of preparing a toner in which the pigment ratio in the toner is varied at a level of 3 to 14%, changing the beam diameter of each toner, outputting an image, and measuring the granularity. The pseudo halftone processing used at this time is the error diffusion method using the weight matrix in the first embodiment. FIG. 12 shows that when the pigment ratio is in the range of 3 to 8% and the beam diameters (D1, D2) in the main and sub-scanning directions on the photosensitive drum 1 are each 40 μm or less, the granularity set this time is 0. It is understood that the condition of 0.3 or less is satisfied.
[0077]
(Examples 7, 8 and Comparative Example 12)
Next, an image output result when the specification of the photosensitive drum 1 is changed in the image forming apparatus of the first embodiment will be described. The measurement of the film thickness d of the charge transport layer (CT layer) of the photoconductor drum 1 is performed by a film thickness meter manufactured by Fischer as described above.
Example 7: Photoreceptor drum having a charge transport layer thickness d of 25 μm
Example 8: Photoconductor drum in which the thickness d of the charge transport layer was 35 μm
Comparative Example 12: Photoreceptor drum with charge transport layer thickness d of 50 μm
[0078]
Table 7 shows the evaluation results of the granularity in Examples 7, 8 and Comparative Example 12. As can be seen from Table 7, when the thickness d of the charge transport layer of the photosensitive drum 1 satisfies the conditions of d <D1 and d <D2, the graininess is reduced even when the error diffusion method is used as the halftone processing. It can be seen that the condition does not deteriorate. Here, D1 and D2 are the beam diameters (diameters) on the photosensitive drum 1 in the main scanning direction and the sub-scanning direction, respectively.
[Table 7]

[0079]
Table 8 summarizes the main experimental conditions and evaluation results in Examples 1 to 8 and Comparative Examples 1 to 12 described above.
[Table 8]

[0080]
(Example 9)
Next, a ninth embodiment in which the configuration of the exposure device (optical writing unit) 3 in the image forming apparatus of the first embodiment is changed will be described.
FIG. 13 is a schematic diagram of the exposure device 3 in the image forming apparatus according to the ninth embodiment. The configuration of the exposure apparatus 3 other than the LD unit is the same as that of the first embodiment. In the ninth embodiment, two 1ch type LD units 310 are used in combination. The beam-like laser light emitted from the LD of each LD unit 310 is superimposed using a mirror 311 and a half mirror 312.
Even when a plurality of LD units 310 are mounted as in the ninth embodiment, it is difficult to make the light amounts of the LDs consistent over a long period of time due to differences in the characteristics of the LDs. In particular, compared to the 2-channel LD unit 301 including the LD array of the first embodiment, there are large variations such as emission current characteristics and temperature dependence because the conditions for forming semiconductor elements do not completely match. A particularly important problem is wavelength. The wavelength difference of the laser light emitted from the LD of each LD unit 310 is not directly related to the variation in the amount of emitted light, but since the spectral sensitivity characteristic of the photoconductor is not constant at all wavelengths, May cause the same problem as variation.
Such a problem does not occur in the image forming apparatus having the configuration of the ninth embodiment. That is, for the same reason as in the first embodiment, it is possible to prevent the occurrence of an abnormal image in which density unevenness occurs on the image due to the wavelength difference between the LD units 310. Further, in the tenth embodiment, a low cost 1ch type LD unit can be used, so that the cost of the exposure apparatus 3 can be reduced.
[0081]
(Example 10)
FIG. 14 is a schematic diagram of the exposure device 3 in the image forming apparatus according to the tenth embodiment. Unlike the first embodiment, the exposure apparatus 3 has a configuration in which two sets of optical units 320 are arranged in parallel with the surface 1 a of the photosensitive drum 1. Each optical unit 320 includes an LD unit 321, a collimating lens 322, a cylindrical lens 323, a polygon mirror 324, and an f-θ lens 325. In the configuration in which the two optical units 320 are arranged in parallel with the surface 1a of the photosensitive drum 1 as in the tenth embodiment, the section in which the laser beam LB scans over the photosensitive drum surface 1a can be shortened. The following advantages can be obtained.
(1) A wide image can be handled. (So-called wide machine)
(2) Since the scanning section of the laser beam is short, the beam diameter can be easily reduced.
(3) The pixel clock can be delayed.
[0082]
On the other hand, as can be seen from FIG. 14, there is a problem that the joint of the laser beam LB appears at the center of the image. When there is a light amount difference between LDs, since the image is located at the center of the image, there is a problem that a difference in density occurs at the joint in an image including a large amount of halftone (medium density) such as a photographic image. is there. Against this problem, when an error diffusion method having no periodic structure is used as the pseudo halftone processing method, the light amount difference does not appear as shading on the image. Therefore, it is possible to realize an image forming apparatus that does not cause the problem of the joint at the center of the image.
[0083]
(Example 11)
In the eleventh embodiment, an LED writing optical system using an LED (light emitting diode) is used for the exposure apparatus. The LED writing optical system has: (1) a reduction in noise due to the absence of a driving unit such as a polygon mirror, and (2) a reduction in the size of the optical unit itself, as compared with the method using an LD in Example 1 or the like. It has the advantage of saving space by making it small. On the other hand, since a large number of light emitting elements are arranged close to each other, there is a problem that thermal and electrical crosstalk similar to that in the first embodiment is likely to occur. Further, in order to drive a large number of light emitting points, it is necessary to mount a plurality of IC chips for driving the LDE. Even with this LED driving IC, it is extremely difficult to match the characteristics in all cases.
Such a problem does not occur in the image forming apparatus having the configuration of the eleventh embodiment. That is, for the same reason as in the first embodiment, it is possible to prevent the occurrence of an abnormal image in which density unevenness is present on an image due to variations in the characteristics of each LED driving IC.
[0084]
As described above, according to the present embodiment, the lengths of one pixel calculated based on the resolution in the main and sub-scanning directions are Lp1 and Lp2, respectively, and the irradiation light on the photosensitive drum surface in the main and sub-scanning directions is set. When the beam diameters are D1 and D2, respectively, conditional expressions D1 <Lp1 and D2 <Lp2 are satisfied. Thus, as described in the first embodiment, it is possible to solve a problem specific to the image forming apparatus using the electrophotographic method. That is, it is possible to solve the problem of deterioration of "granularity" which is an important item in image quality when an error diffusion method having no specific periodic structure is used. This is because limiting the beam diameter to a specific range makes it less likely to be affected by the frequency dependence of the developing electric field, which is a problem in the developing process. The spatial frequency dependence of the development means that the development electric field changes according to the writing pattern (spatial frequency of the image), and as a result, it does not originally exist in the image data after the pseudo halftone processing. This causes a problem that such low-frequency density fluctuation occurs in an image after development. The deterioration of the “granularity” gives a rough impression in an image region that should be uniform, and is a very large cause of image quality deterioration particularly in a pictorial image such as a photograph. In the present embodiment, it is possible to realize an electrophotographic image forming apparatus in which granularity is not deteriorated even if an error diffusion method having no specific periodic structure is used as a pseudo halftone processing method.
Further, by using an error diffusion method having no specific periodic structure as a pseudo halftone processing method, an image forming apparatus having the following advantages (1) and (2) can be realized.
{Circle around (1)} Halftone can be expressed without occurrence of moire.
{Circle around (2)} A halftone can be expressed without the influence of the writing position shift appearing on the image.
Further, according to the present embodiment, by using a toner having a volume average particle diameter of 5.5 μm or less, it is possible to reduce the granularity defined by the above Expression 5 to 3.0 or less. By setting the granularity to 3.0 or less in this way, an image forming apparatus that sufficiently satisfies the requirements of high image quality can be realized, as compared with hard copy devices other than the electrophotographic system such as printing. be able to.
[0085]
According to the present embodiment, the pseudo halftone processing using the error diffusion method is performed. This error diffusion method is a pseudo halftone processing method called an FM screen, and is a method of changing the density of dots (frequency modulation) to reproduce the density of an image. For this reason, it does not have a periodic structure and has the same effects as the above (1) and (2). Furthermore, the error diffusion method distributes the difference between the value of the input image and the processed image for each pixel to peripheral pixels when obtaining a pseudo-halftone processed image from the input image. Good reproducibility of density.
On the other hand, in the dither method, when a difference between the input image and the processed image occurs, the error corresponding to the difference is ignored. Therefore, the dither method is slightly inferior to the error diffusion method from the viewpoint of image density reproducibility. In addition, the blue noise method and the like are slightly inferior to the error diffusion method from the viewpoint of image density reproducibility.
[0086]
Further, according to the present embodiment, it is possible to suppress density unevenness when the exposure apparatus 3 has a plurality of light emitting units (LDs: laser diodes). When a plurality of light emitting portions are provided, there is a problem that density unevenness of a spatial frequency (about 50 to 100 lpi) which can be perceived on an image due to a difference in light amount between the light emitting portions occurs. There is. As described above, the light quantity at each light emitting point is as follows: (1) electrical crosstalk, (2) thermal crosstalk, (3) difference in aging of light emitting element, (4) difference in temperature dependence of light emitting element, For example, a light amount difference occurs. Due to such a light amount difference, density unevenness that does not originally exist on the above-described image occurs. Further, since a plurality of pixels are simultaneously scanned and written, even if the light amounts at the light emitting points are completely matched, the scanning method is different (see FIG. 15). Occurs. Case 1 in FIG. 15 shows a case where a pattern consisting of two dots is written by one scan (scan 1) of a laser beam. Case 2 in FIG. 15 shows a case where the same pattern composed of two dots as in Case 1 is written by two scans (scan 1 and scan 2) of the laser beam.
Even for such a problem, it is effective to combine an error diffusion method having no periodic structure as a pseudo halftone processing method. By employing the error diffusion method having no periodic structure, even if the light amount difference between LDs occurs as described above, an image forming apparatus that does not generate an abnormal image can be lengthened. Can be realized over a period of time. Further, it is possible to realize an image forming apparatus in which an abnormal image does not occur even under use conditions in which environmental changes are large. Further, in a high-speed image forming apparatus in which a plurality of LDs must be used, the above-described image forming apparatus can be realized.
[0087]
In the present embodiment, as the light source of the exposure device 3, a light emitting member in which a plurality of light emitting units are arranged in an array on one chip at an interval of tens to several tens of μm can be used. When such a light emitting member is used, the following effects can be obtained. First, since the light emitting units are extremely close to each other, the light paths of the light sources passing through the optical unit may be substantially the same. This means that the position at which the optical path passes through an optical element (f-θ lens, folding mirror, etc.) disposed in the optical unit is extremely close. For this reason, a plurality of light sources can be used at places (parts) where the characteristics of the optical element are close. Therefore, there are the following advantages: (1) it is easy to match the exposure position of the laser beam on the photoconductor for each light source; and (2) it is easy to match the so-called spot diameter of the laser beam on the photoconductor. Further, in the case of a semiconductor laser diode, the LD array formed on one chip has a feature that the wavelength difference of laser light emitted from each light-emitting portion (light-emitting source) is very small for manufacturing reasons. is there. Since the wavelength difference of the laser beam is small, the phenomenon that the exposure position on the surface of the photosensitive drum shifts for each light emitting portion (light emitting source) is less likely to occur.
A light-emitting member in which light-emitting portions are arranged in an array has the advantages described above, but also has the following disadvantages. In this type of light emitting member, as described above, since the distance between the light emitting units is an interval of tens of μm to several tens of μm, the influence of the light modulation of one light emitting unit due to the heat generation of each light emitting unit causes adjacent light emitting units to emit light. This affects the light intensity as a function of light intensity. So-called thermal crosstalk tends to increase. Further, there is a large electric crosstalk in which the modulation current of the LD causes electric induction in the adjacent light emitting unit. It is effective to combine the error diffusion method having no periodic structure as a pseudo halftone processing method also for these problems of thermal crosstalk and electric crosstalk. By employing the error diffusion method having no periodic structure, an abnormal image (image density in the sub-scanning direction) caused by a light amount difference of each light emitting unit due to crosstalk does not occur on an output image. For this reason, only the advantages of the light emitting members in which the light emitting units are arranged in an array can be utilized, and an image forming apparatus with high positional accuracy can be realized.
[0088]
In the present embodiment, a stacked photoconductor can be used as the photoconductor of the photoconductor drum 1. There are a laminated photoreceptor and a single-layer photoreceptor as the photoreceptor. The laminated photoreceptor has (1) a high sensitivity, and (2) a short time until the charge is canceled after the optical writing ( Responsiveness). On the other hand, in the case of a stacked photoreceptor, since the charge generation layer is present on the bottom side of the photoreceptor, the photocarriers travel a long distance (10 to 30 μm corresponding to the thickness of the charge transport layer) in the charge transport layer. There must be. During this movement, there is a structural problem that the photocarriers spread toward the surface of the photoreceptor due to the Coulomb repulsion between the photocarriers. Due to such diffusion of photocarriers, the electrostatic latent image (charge distribution on the photoconductor) formed on the photoconductor becomes considerably larger than the beam diameter, and the charge contrast is broad. It becomes a shape. The spread of the electrostatic latent image compared to the beam diameter does not cause a serious problem when an AM screen such as a dither method is used as a pseudo halftone processing method. This is because, in the dither method or the like, pseudo halftone processing is often performed in a dot concentration type, and the spread of such an electrostatic latent image affects only the dots located at the peripheral portion among the dots forming the halftone dot, This is because there is no effect on the dots existing inside. However, when an FM screen such as each error diffusion method is used, the influence is great. This is because, in the error diffusion method or the like, dots are often formed discretely, and it can be considered that all dots exist in the peripheral portion. For this reason, the influence of the spread of the electrostatic latent image is greater. As a result, the spread of the electrostatic latent image is the same as the result when the beam diameter is large, and when combined with a pseudo halftone processing method that does not have a periodic structure, there is a problem that graininess deteriorates. cause.
In the present embodiment, the lengths of one pixel calculated based on the resolution in the main and sub-scanning directions are Lp1 and Lp2, respectively, and the beam diameter of the irradiation light on the photosensitive drum surface in the main and sub-scanning directions is respectively When D1 and D2 are set, the conditional expressions D1 <Lp1 and D2 <Lp2 are satisfied. For this reason, it is possible to suppress the deterioration of graininess even when a stacked type photoconductor in which an electrostatic latent image is easily spread and a pseudo halftone processing method having no periodic structure are combined. . Further, the use of a stacked type photoreceptor makes it possible to realize an image forming apparatus equipped with a photoreceptor having high sensitivity and high response speed. It is possible to improve the image forming apparatus.
[0089]
In particular, in the case of using the above-mentioned stacked type photoreceptor, when the thickness of the charge transport layer (CT layer) is d, it is necessary to satisfy the conditions of d <D1 and d <D2. The reason why this condition is necessary can be considered as follows. When the photoreceptor is a stacked photoreceptor, when the photocarriers generated in the charge generation layer pass through the charge transport layer and combine with the charges on the photoreceptor surface, the charge transport layer is charged by electrostatic Coulomb force. A phenomenon occurs in which the photo carrier spreads in the direction of the photoreceptor surface. In addition, a phenomenon occurs in which the photosensitive member moves toward the surface of the photoconductor under the influence of a photocarrier generated in another portion due to electrostatic Coulomb force. As a result of these phenomena, the electrostatic latent image spreads and a sharp latent image cannot be formed. From the viewpoint of the latent image formed on the photoconductor, it is no different from writing with a large beam diameter. Also, the position of the center of gravity of the photo carrier moves from the normal position, which causes noise on the image.
Such an effect increases as the thickness of the charge transport layer increases. When the thickness of the charge transport layer is smaller than the beam diameter (d <D1 and d <D2), the above-mentioned effect is not so large, and the sharpness of the latent image is determined substantially by the beam diameter at which writing is performed. It is thought that. However, when the thickness of the charge transport layer is larger than the beam diameter (d> D1, d> D2), even when the write beam diameter is reduced, the sharpness of the latent image is reduced. Since it is determined by the thickness, the effect of preventing the deterioration of the graininess, which is the object of the present invention, cannot be obtained. That is, when the charge transport layer thickness d satisfies the conditions of d <D1 and d <D2 in the case of using the stacked photoconductor, it is possible to prevent the deterioration of the graininess.
[0090]
Further, the mass ratio of the coloring material (pigment) to the resin constituting the toner used in the image forming apparatus of the present embodiment is preferably in the range of 3.0 to 8.0%. In this case, the deterioration of graininess can be prevented more reliably.
[0091]
In the image forming apparatus of the present embodiment, the resolution is preferably 600 dpi or more. The effects described so far mainly focus on the effect of a halftone portion on an image. Specifically, it was about "granularity" of a portion subjected to the pseudo halftone processing. Regarding the graininess of the halftone area, there was no problem if the resolution of the image forming apparatus was 400 dpi or more. However, with regard to the jaggies of the character portion and the fine lines, a problem may occur when the resolution is about 400 dpi. In particular, when the volume average particle diameter of the toner becomes small, the unevenness of the toner image itself becomes considerably smooth, and the lack of writing resolution is perceived accordingly. In particular, when the volume average particle diameter of the toner is 5.5 μm or less as in this embodiment, jaggies become noticeable at a resolution of 400 dpi, which is not preferable in image quality. Therefore, by increasing the resolution to 600 dpi, such jaggy can be solved and high image quality can be achieved.
[0092]
In this embodiment, the duty ratio of light emission for one pixel is preferably 50% or less. When the duty ratio of the light emission is increased, the writing area per pixel increases. This is because the laser beam scans the surface of the photosensitive drum while the laser is emitting. FIGS. 16 and 17 schematically show how the laser beam is scanned when the duty ratio is 50% and 100%, respectively. The solid lines in each figure show the irradiation spots of the laser beam at the start and end of irradiation for one pixel. As shown in FIG. 16, when the light emission duty ratio for one pixel is 50%, the area over which the laser beam scans the surface of the photosensitive drum during the light emission time is small. On the other hand, when the duty ratio of light emission for one pixel is 100% as shown in FIG. 17, the area for scanning on the photoconductor becomes large. That is, if the duty ratio of the light emission is increased, the writing energy of the laser beam is spread without being concentrated, and the effect of reducing the beam diameter cannot be obtained. When the writing energy of the laser beam is increased, the graininess is reduced when combined with the halftone processing method without a periodic structure for the same reason as the spread of the electrostatic latent image by the photoconductor described above. It is thought to get worse.
Therefore, by setting the duty ratio of light emission for one pixel to 50% or less, exposure energy can be concentrated. Further, even when combined with a pseudo halftone processing method having no periodic structure, it is possible to realize an image forming apparatus in which the graininess does not deteriorate.
[0093]
In the present embodiment, the relationship between the lengths Lp1 and Lp2 of one pixel and the beam diameters D1 and D2 of irradiation light (D1 <Lp1, D2 <Lp2), the error diffusion method, and the toner having a small particle diameter of 5.5 μm or less are used. Is effective in a color image forming apparatus that outputs an image in which toner images of a plurality of colors are superimposed. In this type of color image forming apparatus, it is required to avoid interference due to color overlap. Further, in order to substantially prevent the influence of slight plate deviation or the like from appearing, it is an effective countermeasure not to match the periodic structure of the halftone processing method for each color. When using the dither method, it is often used to shift the so-called screen angle for each corner color. However, when the dither method is used, moire tends to occur, and when a plurality of LDs are used, there is a problem that density unevenness easily occurs due to a difference in LD light amount.
On the other hand, in an image forming apparatus using a pseudo halftone processing method that does not have a periodic structure in the sub-scanning direction, such as the error diffusion method in the present embodiment, as described above, the density unevenness Will not appear. Also, the above-described pseudo-halftone processing that does not have a periodic structure in the sub-scanning direction does not cause a problem with respect to the above-described interference unevenness due to color superposition or color change due to plate misregistration. In addition, it is possible to suppress deterioration in graininess on a color image.
[0094]
In the present embodiment, the relationship between the lengths Lp1 and Lp2 of one pixel and the beam diameters D1 and D2 of irradiation light (D1 <Lp1, D2 <Lp2), the error diffusion method, and the toner having a small particle diameter of 5.5 μm or less are used. Is also effective when a color image is formed by superimposing four color toner images of cyan, magenta, yellow and black. In this case, when the dither method is to be used as the pseudo halftone processing, four dither matrices are prepared and combined with a screen angle in order to prevent interference unevenness due to superposition of four colors and color change due to misregistration. Necessary. However, when the dither method is used, moire tends to occur, and when a plurality of LDs are used, there is a problem that density unevenness easily occurs due to a difference in LD light amount.
On the other hand, in an image forming apparatus that uses a pseudo halftone processing method that does not have a periodic structure in the sub-scanning direction, such as the error diffusion method in the present embodiment, unevenness in density occurs on the image in question. There is no such thing. Also, the above-described pseudo-halftone processing that does not have a periodic structure in the sub-scanning direction does not cause a problem with respect to the above-described interference unevenness due to color superposition or color change due to plate misregistration. In addition, deterioration of graininess on a color image in which four color toner images are superimposed can be suppressed.
[0095]
The configuration of the color image forming apparatus to which the present invention can be applied is not limited to a specific configuration. For example, the present invention can be applied to a tandem type (direct transfer) color image forming apparatus, a tandem type (intermediate transfer) color image forming apparatus, a revolver type color image forming apparatus, and the like.
[0096]
FIG. 18 is a schematic configuration diagram of a so-called tandem type (direct transfer) color image forming apparatus. This color image forming apparatus is provided with four sets of image forming units 17 to 20 along the conveying direction of the recording sheet conveyed by the paper conveying belt 16 wrapped around the tension roller 15. Each of the image forming units 17 to 20 includes the photosensitive drum 1 and the charging device 2 and the like, similarly to the above-described image forming apparatus (FIG. 2), and forms toner images of mutually different colors (cyan, magenta, yellow, and black). To form. The toner image formed on the photosensitive drum 1 of each of the image forming sections 17 to 20 is transferred onto the recording sheet 6 at a transfer position where the photosensitive drum 1 and the transfer roller 21 correspond to each other so as to be sequentially superimposed. You. The recording sheet 6 to which the color image has been transferred is discharged outside the apparatus after the toner image is fixed by the fixing device 8. Note that dirt such as residual toner adhering to the surface of the paper transport belt 16 is cleaned by the belt cleaning device 22.
[0097]
FIG. 19 is a schematic configuration diagram of a so-called tandem type (intermediate transfer) color image forming apparatus. In this color image forming apparatus, four sets of image forming units 17 to 20 are arranged along the direction in which an intermediate transfer belt 23 as an intermediate transfer member wound around a tension roller 15 is stretched. Each of the image forming units 17 to 20 includes the photosensitive drum 1 and the charging device 2 and the like, similarly to the above-described image forming apparatus (FIG. 2), and forms toner images of mutually different colors (cyan, magenta, yellow, and black). To form. The toner images formed on the photosensitive drums 1 of the image forming units 17 to 20 are transferred so as to be sequentially superimposed on the intermediate transfer belt 23 at the transfer positions where the photosensitive drums 1 and the transfer rollers 21 correspond. Is done. The color image transferred onto the intermediate transfer belt 23 is collectively transferred to the recording sheet 6 at a secondary transfer position where a transfer roller 24 for secondary transfer faces. The recording sheet 6 to which the color image has been transferred is discharged outside the apparatus after the toner image is fixed by the fixing device 8. Note that dirt such as residual toner adhered to the surface of the intermediate transfer belt 23 is cleaned by the belt cleaning device 22.
[0098]
FIG. 20 is a schematic configuration diagram of a so-called revolver type color image forming apparatus. In this color image forming apparatus, electrostatic latent images corresponding to different colors (cyan, magenta, yellow, and black) are formed on one photosensitive drum 1. The electrostatic latent images of each color on the photosensitive drum 1 are developed with the toner of the corresponding color as the revolver developing device 40 rotates sequentially. The toner images of the respective colors formed on the photosensitive drum 1 are sequentially superimposed and transferred onto the recording sheet 6 held on the paper transport drum 25 at the transfer position where the transfer roller 26 faces. The recording sheet 6 on which the toner images of all colors have been transferred is discharged outside the apparatus after the toner images are fixed by the fixing device 8.
[0099]
【The invention's effect】
According to the first to eleventh aspects of the present invention, the beam diameters D1 and D2 of the irradiation light on the image carrier in the main and sub scanning directions are each set to the length Lp1 of one pixel calculated from the resolution in the main and sub scanning directions. , Lp2. As a result, even if an error diffusion method that does not easily generate moire and has excellent gradation reproducibility is used, an image portion having a low spatial frequency that is easily affected by the spatial frequency dependence of the developing electric field is less likely to be generated. Therefore, it is possible to suppress the deterioration of the graininess caused by the generation of the low spatial frequency image portion. In particular, by using a toner having a volume average particle diameter of 5.5 μm, there is an effect that a good granularity in which the “granularity” defined by the above equation (5) is 3.0 or less can be achieved.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing a relationship between beam diameters D1 and D2 of a laser beam LB on a photosensitive drum surface and lengths Lp1 and Lp2 of one pixel in an image forming apparatus according to an embodiment of the present invention.
FIG. 2 is a schematic configuration diagram of the image forming apparatus.
FIG. 3 is a block diagram showing a main part of a control system including an image processing unit.
FIG. 4A is a plan view of an optical unit in the exposure apparatus.
(B) is a front view of the optical unit.
FIG. 5 is an explanatory diagram of processing contents in an error diffusion processing unit of the image processing unit.
FIG. 6 is an explanatory diagram of a weight matrix (error diffusion matrix) A used in the first embodiment.
FIG. 7 is an explanatory diagram of a weight matrix (error diffusion matrix) B used in the second embodiment.
FIG. 8 is an explanatory diagram of a weight matrix (error diffusion matrix) C used in the third embodiment.
FIG. 9 is a graph showing an example of digitization of granularity.
FIG. 10 is an explanatory diagram of image data that has been subjected to error diffusion processing.
FIG. 11 is an explanatory diagram showing a change in an “intermediate potential region” depending on a beam spot diameter.
FIG. 12 is a graph showing the relationship between the beam spot diameter and the granularity when the pigment ratio is changed.
FIG. 13A is a plan view of an optical unit in an exposure apparatus according to a ninth embodiment.
(B) is a front view of the optical unit.
FIG. 14A is a plan view of an optical unit in an exposure apparatus according to a tenth embodiment.
(B) is a front view of the optical unit.
FIG. 15 is an explanatory diagram showing a case where the same two-dot pattern is written by scanning two different types of laser beams.
FIG. 16 is an explanatory diagram showing a state of scanning by a laser beam when the duty ratio of light emission is 50%.
FIG. 17 is an explanatory diagram showing a state of scanning with a laser beam when the duty ratio of light emission is 100%.
FIG. 18 is a schematic configuration diagram of a color image forming apparatus to which the present invention can be applied.
FIG. 19 is a schematic configuration diagram of a color image forming apparatus according to another embodiment.
FIG. 20 is a schematic configuration diagram of a color image forming apparatus according to still another embodiment.
[Explanation of symbols]
1 Photoconductor drum
1a Photoreceptor drum surface
2 Charging device
3 Exposure equipment
4 Developing device
5 Transfer device
6 Recording sheet
7 Cleaning device
8 Fixing device
10 Image processing unit
101 MTF filter processing unit
102 γ processing unit
103 pseudo halftone processing section
301 LD (Laser Diode) Unit
LB laser beam

Claims (11)

An image carrier, pseudo-halftone processing means for performing pseudo-halftone processing on input image data to generate output image data, and irradiating the image carrier with beam-shaped irradiation light based on the output image data An exposure device that forms a latent image by performing, and a developing device that develops a latent image on the image carrier,
In the image forming apparatus, wherein the pseudo halftone processing unit performs pseudo halftone processing by an error diffusion method,
The length of one pixel calculated based on the resolution of the irradiation light in the main scanning direction is defined as Lp1, and the length of one pixel calculated based on the resolution of the irradiation light in the sub-scanning direction is defined as Lp2. When the beam diameter of the irradiation light on the carrier in the main scanning direction is D1, and the beam diameter of the irradiation light on the image carrier in the sub-scanning direction is D2, D1 <Lp1 and D2 <Lp2. Meet the relationship,
An image forming apparatus, wherein a volume average particle diameter of a toner in a developer used in the developing device is 5.5 μm or less. An image carrier, pseudo-halftone processing means for performing pseudo-halftone processing on input image data to generate output image data, and irradiating the image carrier with beam-shaped irradiation light based on the output image data An exposure device that forms a latent image by performing, and a developing device that develops a latent image on the image carrier,
The pseudo halftone processing means performs pseudo halftone processing by an error diffusion method;
In the image forming apparatus, wherein the light source in the exposure apparatus has a plurality of light emitting units,
The length of one pixel calculated based on the resolution of the irradiation light in the main scanning direction is defined as Lp1, and the length of one pixel calculated based on the resolution of the irradiation light in the sub-scanning direction is defined as Lp2. When the beam diameter of the irradiation light on the carrier in the main scanning direction is D1, and the beam diameter of the irradiation light on the image carrier in the sub-scanning direction is D2, D1 <Lp1 and D2 <Lp2. Meet the relationship,
An image forming apparatus, wherein a volume average particle diameter of a toner in a developer used in the developing device is 5.5 μm or less. The image forming apparatus according to claim 2,
An image forming apparatus, wherein the light source is a light emitting member in which a plurality of light emitting units are arranged in an array on one chip. The image forming apparatus according to claim 1, wherein
The image carrier is a laminated photoconductor having a charge transport layer formed on a charge generation layer,
An image forming apparatus which satisfies d <D1 and d <D2 when the thickness of the charge transport layer is d. The image forming apparatus according to claim 1, wherein
An image forming apparatus, wherein the ratio of the mass of the coloring material to the resin constituting the toner is 3.0 to 8.0%. The image forming apparatus according to claim 1, wherein
An image forming apparatus, wherein the resolution is at least 600 dpi. The image forming apparatus according to claim 1, wherein
An image forming apparatus, wherein a duty ratio of the irradiation light to each pixel is 50% or less. The image forming apparatus according to claim 1, wherein
An image forming apparatus, wherein a pixel clock frequency which is a reciprocal of a writing time of the irradiation light for each pixel is 30 MHz. The image forming apparatus according to any one of claims 1 to 8,
A color image is formed by superimposing a plurality of toner images of different colors on a transfer body,
An image forming apparatus, wherein at least one color toner image of the plurality of toner images is a toner image obtained by forming a latent image with the exposure device and developing the latent image with the developing device. The image forming apparatus according to claim 9,
An image forming apparatus, wherein the plurality of toner images are four-color toner images of cyan, magenta, yellow, and black. The image forming apparatus according to claim 10,
Of the four color toner images, three color toner images of cyan, magenta and yellow are toner images obtained by forming a latent image with the exposure device and developing the latent image with the developing device. An image forming apparatus comprising:

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