JP4363755B2

JP4363755B2 - Image forming apparatus and image forming method

Info

Publication number: JP4363755B2
Application number: JP2000199096A
Authority: JP
Inventors: 順二松澤
Original assignee: Ricoh Co Ltd
Current assignee: Ricoh Co Ltd
Priority date: 2000-06-30
Filing date: 2000-06-30
Publication date: 2009-11-11
Anticipated expiration: 2020-06-30
Also published as: JP2002019189A

Description

【０００１】
【発明の属する技術分野】
本発明は、画像形成装置及び画像形成方法、特に画像部分の明度を調整して出力画像の高品位を得る技術に関する。
【０００２】
【従来の技術】
画像形成装置の中で、高速かつ低騒音の装置の一つとして、電子写真方式を採用したものが知られている。複写機やレーザービームプリンタなどの電子写真方式の一般的な画像形成装置においては、像担持体として、表面に感光層を有するドラム状やベルト状の感光体を備えている。そして、感光体の周囲には、感光層の移動方向に沿って順に、感光体を帯電させて感光性を与える帯電装置、感光体を露光して静電潜像を作る露光装置、潜像をトナーなどの現像剤で現像する現像装置、現像された像を記録媒体に転写する転写分離装置、感光体についたトナーを除去するクリーニング装置、感光体を除電する除電装置が配置されている。さらに、記録媒体の搬送方向における感光体の下流側には、感光ドラムから記録媒体に転写されたトナー像を融着して定着させる定着装置が設けられている。記録媒体としては、一般的に紙などが使用されている。
【０００３】
階調画像を形成する画像形成装置及び画像形成方法に関する従来技術の一例として、特開平ー号公報１１−１７９４７号公報が挙げられる。この公報によれば、出力特性を視覚特性に適合させることによって出力画像の高品位化を図ることを目的として、明度の差が略一定である複数の濃度の画素により記録媒体上に階調画像を形成する構成が開示されている。更に、この公報では、具体的には、所定の濃度の濃度補正用画像を形成してその濃度を測定する濃度測定手段と、測定した濃度を明度に変換する変換手段と、変換に係る明度を均等に分割して分割に係る各境界に対応する濃度を算出して算出した濃度の情報を保持する濃度特性設定手段と、保持された濃度の情報を参照して画像信号の濃度を分割に係る各境界に対応する濃度のいずれかに変換する濃度変換手段と、濃度を変換した画像信号に基づいて記録媒体上に階調画像を形成する階調画像形成手段と、を備えた画像形成装置が開示されている。なお、この公報には記録媒体の明度についての考慮は払われていない。
【０００４】
【発明が解決しようとする課題】
しかしながら、記録媒体として一般的に使われている紙は、各メーカーにより若干ではあるが明度に差が有る。また、近年、紙のリサイクル化が進んでいるため、この傾向が顕著化している。この様な紙（例えば、やや黄ばんだ紙）を記録媒体として使用すると、あるいは、この様な紙が混在した状態で使用すると、画像部分の明度が一定であるのに対し、背景部分（記録媒体の地肌部分）の明度が変動するため、画像部分と背景部分の明度の比が記録媒体毎に異なってしまい、記録媒体複数枚にまたがる書類を作成した場合、大変見にくいものとなってしまい、目の疲労感が増大すると言う課題があった。
【０００５】
本発明は、上記課題に鑑みてなされたものであり、記録媒体の明度に合わせて画像部分の最大濃度（例えば、一番黒い画像部分）を変化させて、画像の最大濃度の明度と記録媒体の明度の比を略一定にし、また、階調性画像を形成する場合に、画像の最大濃度の明度と記録媒体の明度の間を各階調間の明度の差が略一定となるように分割すること等により、出力画像の高品位化を図ることを目的とする。
【０００６】
【課題を解決するための手段】
前記課題を解決するために、本発明は主として次のような構成を採用する。
【０００７】
第１の手段は、搬送される記録媒体の反射率を測定する反射率測定手段と、前記反射率から求められる明度に基づいて、前記記録媒体の明度指数を算出する記録媒体明度指数算出手段と、前記記録媒体の明度指数と、所定の基準値として設定された前記記録媒体の基準明度指数及び画像の最大濃度の基準明度指数とに基づいて、前記画像の最大濃度の明度指数を算出する最大濃度明度指数算出手段と、前記画像の階調性の有無を決定する階調性画像決定手段と、前記画像の階調性の有無に応じて、単一の濃度の画像を形成する場合には、前記画像の最大濃度の明度指数をもとに所望の濃度に変換する一方で、前記階調性をもつ画像を形成する場合には、前記記録媒体の明度指数と前記画像の最大濃度の明度指数とをもとに各階調の明度指数を設定し、当該設定された各階調の明度指数に対応した明度から得られる濃度に変換する濃度変換手段とを備えたことを特徴とする。
【０００８】
第２の手段は、第１の手段において、前記濃度変換手段は、前記階調性をもつ画像を形成する場合に、前記記録媒体の色に最も近い階調と前記記録媒体の有する階調との濃度差を、他の階調間の濃度差よりも大とすることを特徴とする。
【０００９】
第３の手段は、第１の手段において、当該画像形成装置は、モノクロ又はカラーの電子写真プロセスに基づく画像形成を行うことを特徴とする。
【００１０】
第４の手段は、搬送される記録媒体の反射率を測定する反射率測定処理と、前記反射率測定処理で測定した前記反射率から求められる明度に基づいて、前記記録媒体の明度指数を算出する記録媒体明度指数算出処理と、前記記録媒体明度指数算出処理で算出した前記記録媒体の明度指数と、所定の基準値として設定された前記記録媒体の基準明度指数及び画像の最大濃度の基準明度指数とに基づいて、前記画像の最大濃度の明度指数を算出する最大濃度明度指数算出処理と、前記画像の階調性の有無を決定する階調性画像決定処理と、前記階調性画像決定処理で決定した前記画像の階調性の有無に応じて、単一の濃度の画像を形成する場合には、前記最大濃度明度指数算出処理で算出した前記画像の最大濃度の明度指数をもとに所望の濃度に変換する一方で、前記階調性をもつ画像を形成する場合には、前記記録媒体明度指数算出処理で算出した前記記録媒体の明度指数と前記画像の最大濃度の明度指数とをもとに各階調の明度指数を設定し、当該設定された各階調の明度指数に対応した明度から得られる濃度に変換する濃度変換処理とを実行することを特徴とする。
【００１１】
第５の手段は、第４の手段において、前記濃度変換処理は、前記階調性をもつ画像を形成する場合に、前記記録媒体の色に最も近い階調と前記記録媒体の有する階調との濃度差を、他の階調間の濃度差よりも大とすることを特徴とする。
【００１９】
【発明の実施の形態】
本発明の実施形態に係る画像形成装置について図面を用いて以下説明する。図１、図２は、本発明の実施形態に係る画像形成装置の構成例を示す図であり、画像形成装置は、感光体１、帯電装置２、露光装置３、現像装置４、転写分離装置５、クリーニング装置６、除電装置７、明度測定装置８、定着装置９から構成される。ここで、露光装置３は、レーザ制御装置１０、レーザ発生装置１１、ポリゴンミラー１２、ｆθレンズを含む光学系１３を備えている。
【００２０】
次に、画像形成装置の各構成要素の機能について説明すると、帯電装置２は、感光体１を帯電させて感光性を与え、露光装置３は、記録媒体１５に形成する画像情報に基づいて、レーザ発生器１１よりレーザビーム１４を出力し、ポリゴンミラー１２やｆθレンズを含む光学系１３により感光体１表面にレーザビーム１４を結像させながら走査し、静電潜像を形成する。この時、レーザ制御装置１０は画像情報に基づいてレーザービーム１４の出力を制御している。
【００２１】
また、現像装置４は、露光装置３により形成された感光体１上の静電潜像を現像剤などにより可視像に現像する。転写分離装置５は、現像された像を記録媒体１５に転写し、転写後、記録媒体１５を感光体１から分離する。クリーニング装置６は、転写後に感光体１上に残った現像剤を除去する。除電装置７は、感光体１上に残っている電荷を除電する。定着装置９は、記録媒体１５の搬送方向における感光体１の下流側に設置されていて、感光体１から記録媒体１５に転写された転写像を融着し定着させる。
【００２２】
また、明度測定装置８は、記録媒体１５の搬送方向における感光体１の上流側に設置されていて、露光プロセスが始まる前に、搬送されてくる記録媒体１５毎の明度を測定しその情報をレーザ制御装置１０に送る。明度測定装置８は給紙テーブル内に有っても良いし、給紙テーブルから転写分離装置５までの給紙経路途中に有っても良い。
【００２３】
以上説明した画像形成装置は基本的な概略構成を示したものであって、実際上の具体的な画像形成装置は前記概略構成に種々の他の構成要素を付加、具備したものであり、本発明の画像形成装置は実際に動作させるに必要な具体的構成を具備したものを当然のことながら対象とするものである。
【００２４】
次に、画像の最大濃度及び各階調の濃度を決定するプロセスを説明する。ここで、画像濃度の決定において、記録媒体に形成する画像部分の濃度に階調性を持たせることなく単一の最大濃度で画像を形成する場合と、画像部分の濃度に階調性を持たせて最大濃度から最小濃度までの濃度で画像（階調性画像）を形成する場合があり、本発明においては両者の場合を含めて画像形成を実行しようとするものである。
【００２５】
図３は画像の最大濃度又は各階調の濃度を決定する手順を示す図である。明度測定装置８では、先ず、光センサなどにより記録媒体１５の反射率を測定する（Ｐ１）。次いで、記録媒体１５の光反射率ρから、ＣＩＥ１９７６のＬ^＊ａ^＊ｂ^＊色空間による明度指数Ｌ_Ｂ ^＊を算出する（Ｐ２）。
【００２６】
明度指数Ｌ^＊と反射率ρの関係は、
Ｌ^＊＝（１１６×ρ^１／３）−１６ …（１）
のように式（１）で表現される。また、明度測定装置８で測定された記録媒体１５の明度指数Ｌ_Ｂ ^＊はレーザ制御装置１０に送られる（Ｐ３）。
【００２７】
レーザ制御装置１０では、記録媒体における明度指数での画像部分の見やすさの観点で予め設定されている基準値（記録媒体の基準明度指数Ｌ_ＢＲ ^＊、画像の最大濃度の基準明度指数Ｌ_ＰＲ ^＊）をもとに、次の式（２）により、画像の最大濃度の明度指数Ｌ_Ｐ ^＊を算出する（Ｐ４）。
【００２８】
Ｌ_ＢＲ ^＊：Ｌ_ＰＲ ^＊＝Ｌ_Ｂ ^＊：Ｌ_Ｐ ^＊ …（２）
例えば、基準値がＬ_ＢＲ ^＊＝９４、Ｌ_ＰＲ ^＊＝１６と設定されていた場合、Ｌ_Ｂ ^＊＝９０のときＬ_Ｐ ^＊＝１５．３２（例１）、Ｌ_Ｂ ^＊＝８５のときＬ_Ｐ ^＊＝１４．４７（例２）、Ｌ_Ｂ ^＊＝８０のときＬ_Ｐ ^＊＝１３．６２（例３）となる。これらの数値は図４に示す通りであって、図４では画像部分の階調数を８とした場合の画像部分の明度指数を示し、階調０が画像部分の最大明度指数、即ち最も黒い部分を表し、階調７が記録媒体の明度指数を表している。
【００２９】
換言すると、図４において、階調７における基準値９４は記録媒体の明度指数を表し、階調０における基準値１６は画像部分の最大濃度の明度指数を表していて、この記録媒体と画像部分の明度指数関係を画像の見やすさの基準として、例えば、記録媒体の明度指数が、例１として示されているように９０のときに、画像部分の最大濃度の明度指数が１５．３２であれば良いことを図示しているのである。
【００３０】
図４に示すように、今回、基準値をＬ_ＢＲ ^＊＝９４、Ｌ_ＰＲ ^＊＝１６と設定したが、基準値はこの値に限ったものではなく、条件、状況に応じて任意に設定可能である。この様な算出データの複数種類を準備しておいて、これらをメモリなどの記憶装置に記憶させておき、記録媒体１５の明度指数Ｌ_Ｂ ^＊の値に対応して、記憶装置から読み出すようにしても良い。このように、記録媒体１５の明度がばらついても、記録媒体１５の明度と画像の最大濃度に対応した明度の比を一定に保つようにする。
【００３１】
次に、階調性を有する画像を形成するのか否かを決定し（Ｐ５）、階調性画像でない場合（階調性画像で画像を形成するのではなくて単一の最大濃度で画像を形成する場合）には、式（２）により算出された明度指数Ｌ_Ｐ ^＊は、式（１）をもとに反射率ρに変換される（Ｐ６）。更に、反射率ρは次に示す式（３）によって画像濃度Ｄに変換され（Ｐ７）、画像濃度に対応した出力となるようにレーザビーム１４出力が制御されて（Ｐ８）、最終的に記録媒体に所望濃度の画像が形成される（Ｐ９）。
【００３２】
Ｄ＝−ｌｏｇρ …（３）
ここにおいて、レーザビーム出力制御（Ｐ８）から記録媒体への所望濃度の画像形成に至る手順については、図示していないが、感光体表面への静電潜像の形成、可視像への現像、記録媒体への転写及び定着が実行されるものである。
【００３３】
以上説明したような一連のプロセスにより画像が形成される。本実施形態の構成例では、記録媒体１５の明度指数Ｌ^＊の算出を明度測定装置８で行っているが、この機能をレーザ制御装置１０に持たせても良い。
【００３４】
次に、階調性を有する画像を形成するのか否かを決定（Ｐ５）において、階調性画像を形成する場合には、式（１）で算出された記録媒体１５の明度指数Ｌ_Ｂ ^＊と、式（２）で算出された画像の最大濃度の明度指数Ｌ_Ｐ ^＊をもとに各階調の明度指数を設定する（Ｐ１０）。
【００３５】
例えば、階調数が８階調の場合、Ｌ_Ｂ ^＊とＬ_Ｐ ^＊の間を７分割し、図４に示すように、各階調間の明度が略等間隔になるように設定する。図４では画像の最大濃度の階調を０、記録媒体の階調を７としている。この時、各階調と明度の関係は図５のように直線になる。図４、図５の例では８階調であるが、ｎ階調の場合はＬ_Ｂ ^＊とＬ_Ｐ ^＊の間を（ｎ−１）分割すれば良い。
【００３６】
更に、以上のように設定された各階調の明度指数は、式（１）をもとに反射率に変換される（Ｐ１１）。各階調と反射率の関係を図６、図７に示す。さらに、反射率は、前記Ｐ７に示したと同様に各階調の濃度に変換されて（Ｐ１２）、画像濃度に対応した出力となるようにレーザビーム１４出力を制御し（Ｐ８）、記録媒体へ画像が形成される（Ｐ９）のである。本実施形態の構成例においても、記録媒体１５の明度指数Ｌ^＊の算出を明度測定装置８に代えて、レーザ制御装置１０に持たせても良い。
【００３７】
本実施形態では、階調性のある画像形成の場合に、階調間の濃度差が一定であって人間の眼で見て各階調間濃度差がリニアとなるように構成した例を示したが、この例に限らず、記録媒体の色に最も近い色、即ち図４の例では階調６の濃度については、階調６の画像と記録媒体との識別性の観点で、階調６と階調７の濃度差を他の階調の濃度差よりも大とするような階調画像としても良い。即ち、図４の例で、階調０と階調６までの濃度差は一定の規定値とし、階調６と階調７の濃度差は前記規定値よりも大とする画像形成である。
【００３８】
更に、記録媒体が白地のもの乃至白地に若干の着色のものであって画像が黒色の場合に限らず、記録媒体が黒色であって画像が白色乃至灰色の場合のものについても、本発明は適用可能である。
【００３９】
以上説明した本実施形態においては、モノクロの電子写真プロセスを例として説明したが、カラーの電子写真プロセスにおいても本発明を適用できることは言うまでもなく、さらには、インクジェットプリンタなどの画像形成装置にも本発明を適用できることは明らかである。
【００４０】
【発明の効果】
本発明によれば、記録媒体毎の明度に応じて画像の最大濃度と、各階調の濃度を変更することで、記録媒体毎の明度にばらつきが生じた場合でも、画像の最大濃度の明度と背景部分の明度の比を一定にし、かつ、各階調間の明度の差を一定にすることができ、良好な階調画像を提供することが可能となる。
【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の概略的な全体構成を示す図である。
【図２】本実施形態に係る画像形成装置における露光装置の構成を示す図である。
【図３】本実施形態における画像の最大濃度と各階調濃度を決定する手順を示す図である。
【図４】画像の各階調と明度の関係を示す図である。
【図５】図４に示す各階調と明度の関係を示すグラフである。
【図６】画像の各階調と反射率の関係を示す図である。
【図７】図６に示す各階調と反射率の関係を示すグラフである。
【符号の説明】
１感光体
２帯電装置
３露光装置
４現像装置
５転写分離装置
６クリーニング装置
７除電装置
８明度測定装置
９定着装置
１０レーザ制御装置
１１レーザ発生装置
１２ポリゴンミラー
１３光学系
１４レーザビーム
１５記録媒体[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus and an image forming method, and more particularly to a technique for obtaining high quality output images by adjusting the brightness of image portions.
[0002]
[Prior art]
Among image forming apparatuses, one that employs an electrophotographic system is known as one of high-speed and low-noise apparatuses. In general electrophotographic image forming apparatuses such as copying machines and laser beam printers, a drum-like or belt-like photoreceptor having a photosensitive layer on the surface is provided as an image carrier. Around the photosensitive member, in order along the moving direction of the photosensitive layer, a charging device that charges the photosensitive member to impart photosensitivity, an exposure device that exposes the photosensitive member to create an electrostatic latent image, and a latent image A developing device that develops with a developer such as toner, a transfer separation device that transfers the developed image to a recording medium, a cleaning device that removes toner on the photoconductor, and a static elimination device that neutralizes the photoconductor are arranged. Further, a fixing device for fusing and fixing the toner image transferred from the photosensitive drum to the recording medium is provided on the downstream side of the photosensitive member in the recording medium conveyance direction. In general, paper or the like is used as the recording medium.
[0003]
As an example of the prior art relating to an image forming apparatus and an image forming method for forming a gradation image, JP-A-11-17947 can be cited. According to this publication, in order to improve the quality of an output image by adapting the output characteristic to the visual characteristic, a gradation image is formed on a recording medium by pixels having a plurality of densities whose brightness differences are substantially constant. The structure which forms is disclosed. Further, in this publication, specifically, a density measuring means for forming a density correction image having a predetermined density and measuring the density, a converting means for converting the measured density into lightness, and a lightness relating to the conversion. A density characteristic setting unit that holds the density information calculated by equally dividing and calculating the density corresponding to each boundary related to the division, and the density of the image signal related to the division by referring to the held density information An image forming apparatus comprising: density conversion means for converting to one of densities corresponding to each boundary; and gradation image forming means for forming a gradation image on a recording medium based on an image signal whose density has been converted. It is disclosed. Note that this publication does not consider the brightness of the recording medium.
[0004]
[Problems to be solved by the invention]
However, the paper generally used as a recording medium has a slight difference in brightness depending on the manufacturer. In recent years, since paper recycling is progressing, this tendency is remarkable. When such paper (for example, slightly yellowed paper) is used as a recording medium, or when such paper is used in a mixed state, the brightness of the image portion is constant while the background portion (recording medium) The brightness ratio of the image portion and the background portion varies from recording medium to recording medium. When a document that spans multiple recording media is created, it becomes very difficult to see. There was a problem that the feeling of fatigue increased.
[0005]
The present invention has been made in view of the above problems, and the maximum density of an image portion (for example, the darkest image portion) is changed in accordance with the brightness of the recording medium, whereby the brightness of the maximum density of the image and the recording medium are changed. When forming a gradation image, the brightness of the maximum density of the image and the brightness of the recording medium are divided so that the difference in brightness between the gradations is substantially constant. The purpose of this is to improve the quality of the output image.
[0006]
[Means for Solving the Problems]
In order to solve the above problems, the present invention mainly adopts the following configuration.
[0007]
The first means is a reflectance measuring means for measuring the reflectance of the recording medium being conveyed, and a recording medium brightness index calculating means for calculating the brightness index of the recording medium based on the brightness obtained from the reflectance. , the maximum said the lightness index of the recording medium, based on the reference lightness index maximum density of the reference brightness index and the image of the set the recording medium as a predetermined reference value, calculates a brightness index of the maximum density of the image In the case of forming an image having a single density according to the density lightness index calculating means, the gradation image determining means for determining the presence or absence of gradation of the image, and the presence or absence of the gradation of the image When the image having the gradation property is formed while converting to the desired density based on the brightness index of the maximum density of the image, the brightness index of the recording medium and the brightness of the maximum density of the image The brightness index of each gradation based on the index Constant, and is characterized in that a density conversion means for converting the density obtained from the brightness corresponding to the luminosity index of the set gradation.
[0008]
According to a second means, in the first means, the density conversion means, when forming an image having the gradation, a gradation closest to the color of the recording medium and a gradation possessed by the recording medium The density difference is made larger than the density difference between other gradations .
[0009]
A third means is characterized in that, in the first means, the image forming apparatus performs image formation based on a monochrome or color electrophotographic process .
[0010]
The fourth means calculates the brightness index of the recording medium based on the reflectance measurement process for measuring the reflectance of the recording medium being conveyed and the brightness obtained from the reflectance measured by the reflectance measurement process. Recording medium brightness index calculation process, the recording medium brightness index calculated by the recording medium brightness index calculation process, the reference brightness index of the recording medium set as a predetermined reference value, and the reference brightness of the maximum density of the image A maximum density lightness index calculation process that calculates a lightness index of the maximum density of the image based on the index, a gradation image determination process that determines whether or not the image has gradation, and the gradation image determination In the case of forming an image having a single density according to the presence or absence of gradation of the image determined in the processing, based on the brightness index of the maximum density of the image calculated in the maximum density brightness index calculation process. To the desired concentration On the other hand, when forming an image having the gradation, each level is calculated based on the brightness index of the recording medium calculated by the recording medium brightness index calculation process and the brightness index of the maximum density of the image. A density conversion process is performed in which a tone brightness index is set and density is converted to a density obtained from the brightness corresponding to the set brightness index of each gradation .
[0011]
According to a fifth means, in the fourth means, when the density conversion process forms an image having the gradation, the gradation closest to the color of the recording medium and the gradation of the recording medium The density difference is made larger than the density difference between other gradations .
[0019]
DETAILED DESCRIPTION OF THE INVENTION
An image forming apparatus according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 and FIG. 2 are diagrams showing a configuration example of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus includes a photoreceptor 1, a charging device 2, an exposure device 3, a developing device 4, and a transfer separation device. 5, a cleaning device 6, a static elimination device 7, a brightness measurement device 8, and a fixing device 9. Here, the exposure apparatus 3 includes an optical system 13 including a laser control apparatus 10, a laser generation apparatus 11, a polygon mirror 12, and an fθ lens.
[0020]
Next, the function of each component of the image forming apparatus will be described. The charging device 2 charges the photosensitive member 1 to give photosensitivity, and the exposure device 3 uses the image information formed on the recording medium 15 based on the image information. A laser beam 14 is output from the laser generator 11 and scanned while forming an image of the laser beam 14 on the surface of the photoreceptor 1 by an optical system 13 including a polygon mirror 12 and an fθ lens, thereby forming an electrostatic latent image. At this time, the laser controller 10 controls the output of the laser beam 14 based on the image information.
[0021]
The developing device 4 develops the electrostatic latent image on the photoreceptor 1 formed by the exposure device 3 into a visible image using a developer or the like. The transfer separation device 5 transfers the developed image to the recording medium 15, and after the transfer, separates the recording medium 15 from the photoreceptor 1. The cleaning device 6 removes the developer remaining on the photoreceptor 1 after the transfer. The neutralization device 7 neutralizes the charge remaining on the photoreceptor 1. The fixing device 9 is installed on the downstream side of the photosensitive member 1 in the conveyance direction of the recording medium 15, and fuses and fixes the transfer image transferred from the photosensitive member 1 to the recording medium 15.
[0022]
Further, the brightness measuring device 8 is installed on the upstream side of the photosensitive member 1 in the transport direction of the recording medium 15, and measures the brightness of each transported recording medium 15 before the exposure process starts and obtains the information. This is sent to the laser controller 10. The lightness measuring device 8 may be in the paper feed table or in the middle of the paper feed path from the paper feed table to the transfer separation device 5.
[0023]
The image forming apparatus described above shows a basic schematic configuration, and a practical specific image forming apparatus is obtained by adding various other components to the schematic configuration. The image forming apparatus according to the present invention is naturally intended for an apparatus having a specific configuration necessary for actual operation.
[0024]
Next, a process for determining the maximum density of the image and the density of each gradation will be described. Here, in determining the image density, there is a case where an image is formed with a single maximum density without giving gradation to the density of the image portion formed on the recording medium, and a case where the density of the image portion has gradation. In some cases, an image (gradation image) is formed at a density from the maximum density to the minimum density. In the present invention, image formation is performed including both cases.
[0025]
FIG. 3 is a diagram showing a procedure for determining the maximum density of an image or the density of each gradation. In the lightness measuring device 8, first, the reflectance of the recording medium 15 is measured by an optical sensor or the like (P1). Next, from the light reflectance ρ of the recording medium 15, a lightness index L _B ^* in the L ^* a ^* b ^* color space of CIE 1976 is calculated (P2).
[0026]
The relationship between the brightness index L ^* and the reflectance ρ is
L ^* = (116 × ρ ^1/3 ) −16 (1)
It is expressed by equation (1) as follows. Further, the lightness index L _B ^* of the recording medium 15 measured by the lightness measuring device 8 is sent to the laser control device 10 (P3).
[0027]
In the laser control apparatus 10, a reference value (reference brightness index L _BR ^{* of} the recording medium, reference brightness index L _PR ^* of the maximum density of the image) set in advance from the viewpoint of the visibility of the image portion in the brightness index of the recording medium ^. ), The lightness index L _P ^* of the maximum density of the image is calculated by the following equation (2) (P4).
[0028]
L _BR ^* : L _PR ^* = L _B ^* : L _P ^* (2)
For example, when the reference value is set as L _BR ^* = 94 and L _PR ^* = 16, L _P ^* = 15.32 (example 1) when L _B ^* = 90, and L when L _B ^* = 85 _{When P} ^* = 14.47 (Example 2) and L _B ^* = 80, L _P ^* = 13.62 (Example 3). These numerical values are as shown in FIG. 4. FIG. 4 shows the brightness index of the image portion when the number of gradations of the image portion is 8, and the gradation 0 is the maximum brightness index of the image portion, that is, the blackest The gradation 7 represents the brightness index of the recording medium.
[0029]
In other words, in FIG. 4, the reference value 94 at gradation 7 represents the lightness index of the recording medium, and the reference value 16 at gradation 0 represents the lightness index of the maximum density of the image portion. For example, when the brightness index of the recording medium is 90 as shown in Example 1, the brightness index of the maximum density of the image portion is 15.32. It shows what should be done.
[0030]
As shown in FIG. 4, the reference values are set to L _BR ^* = 94 and L _PR ^* = 16 this time, but the reference values are not limited to these values, and can be arbitrarily set according to conditions and situations. It is. A plurality of types of such calculation data are prepared, stored in a storage device such as a memory, and read out from the storage device in accordance with the value of the lightness index L _B ^* of the recording medium 15. May be. Thus, even if the brightness of the recording medium 15 varies, the ratio of the brightness corresponding to the maximum density of the image and the brightness of the recording medium 15 is kept constant.
[0031]
Next, it is determined whether or not an image having gradation is to be formed (P5), and if it is not a gradation image (not forming an image with a gradation image, the image is formed with a single maximum density). In the case of forming), the lightness index L _P ^* calculated by the equation (2) is converted into the reflectance ρ based on the equation (1) (P6). Further, the reflectance ρ is converted into an image density D by the following equation (3) (P7), the output of the laser beam 14 is controlled so as to be an output corresponding to the image density (P8), and finally recording is performed. An image having a desired density is formed on the medium (P9).
[0032]
D = −logρ (3)
Here, although the procedure from laser beam output control (P8) to image formation of a desired density on the recording medium is not shown, formation of an electrostatic latent image on the surface of the photoreceptor, development to a visible image Then, transfer and fixing to the recording medium are executed.
[0033]
An image is formed by a series of processes as described above. In the configuration example of the present embodiment, the lightness index L ^* of the recording medium 15 is calculated by the lightness measuring device 8, but this function may be provided to the laser control device 10.
[0034]
Next, when determining whether or not to form an image having gradation (P5), in the case of forming a gradation image, the lightness index L _B ^* of the recording medium 15 calculated by Expression (1) is used ^. Then, the brightness index of each gradation is set based on the brightness index L _P ^* of the maximum density of the image calculated by the expression (2) (P10).
[0035]
For example, when the number of gradations is 8, gradation between L _B ^* and L _P ^* is divided into seven, and the brightness between the gradations is set to be approximately equal as shown in FIG. In FIG. 4, the gradation of the maximum density of the image is 0, and the gradation of the recording medium is 7. At this time, the relationship between each gradation and lightness is a straight line as shown in FIG. In the example of FIGS. 4 and 5, there are 8 gradations, but in the case of n gradations, it is only necessary to divide (L−1) between L _B ^* and L _P ^* .
[0036]
Further, the lightness index of each gradation set as described above is converted into reflectance based on the equation (1) (P11). The relationship between each gradation and the reflectance is shown in FIGS. Further, the reflectance is converted into the density of each gradation in the same manner as shown in P7 (P12), and the output of the laser beam 14 is controlled so that the output corresponds to the image density (P8), and the image is recorded on the recording medium. Is formed (P9). Also in the configuration example of the present embodiment, the lightness index L ^* of the recording medium 15 may be calculated by the laser control device 10 instead of the lightness measuring device 8.
[0037]
In the present embodiment, in the case of forming an image with gradation, an example in which the density difference between gradations is constant and the density difference between gradations is linear when viewed by the human eye is shown. However, not limited to this example, the color closest to the color of the recording medium, that is, the density of gradation 6 in the example of FIG. And a gradation image in which the density difference between gradations 7 is larger than the density difference between other gradations. That is, in the example of FIG. 4, the image formation is such that the density difference between the gradation 0 and the gradation 6 is a constant specified value, and the density difference between the gradation 6 and the gradation 7 is larger than the specified value.
[0038]
Furthermore, the present invention is not limited to the case where the recording medium is white or slightly colored on a white background and the image is black, but the present invention also applies to a case where the recording medium is black and the image is white to gray. Applicable.
[0039]
In the present embodiment described above, a monochrome electrophotographic process has been described as an example. However, it goes without saying that the present invention can also be applied to a color electrophotographic process, and further to an image forming apparatus such as an inkjet printer. It is clear that the invention can be applied.
[0040]
【The invention's effect】
According to the present invention, by changing the maximum density of an image and the density of each gradation according to the brightness of each recording medium, even when the brightness of each recording medium varies, The lightness ratio of the background portion can be made constant, and the lightness difference between the gradations can be made constant, so that a good gradation image can be provided.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a schematic overall configuration of an image forming apparatus according to an embodiment of the present invention.
FIG. 2 is a view showing a configuration of an exposure apparatus in the image forming apparatus according to the present embodiment.
FIG. 3 is a diagram showing a procedure for determining the maximum image density and each gradation density in the present embodiment.
FIG. 4 is a diagram illustrating a relationship between each gradation and brightness of an image.
5 is a graph showing the relationship between each gradation and brightness shown in FIG.
FIG. 6 is a diagram illustrating a relationship between each gradation of an image and reflectance.
7 is a graph showing the relationship between each gradation shown in FIG. 6 and reflectance.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Photoconductor 2 Charging device 3 Exposure device 4 Development device 5 Transfer separation device 6 Cleaning device 7 Static elimination device 8 Lightness measurement device 9 Fixing device 10 Laser control device 11 Laser generator 12 Polygon mirror 13 Optical system 14 Laser beam 15 Recording medium

Claims

A reflectance measuring means for measuring the reflectance of the recording medium being conveyed;
A recording medium brightness index calculating means for calculating a brightness index of the recording medium based on the brightness determined from the reflectance;
Maximum concentration the a lightness index of the recording medium, based on the reference lightness index maximum density of the reference brightness index and the image of the set the recording medium as a predetermined reference value, calculates a brightness index of the maximum density of the image Lightness index calculation means,
A gradation image determining means for determining presence or absence of gradation of the image;
When an image having a single density is formed according to the presence or absence of the gradation of the image, the gradation is converted to a desired density based on the brightness index of the maximum density of the image. In the case of forming an image having, the lightness index of each gradation is set based on the lightness index of the recording medium and the lightness index of the maximum density of the image , and corresponds to the lightness index of the set gradation. Density conversion means for converting to a density obtained from the brightness,
An image forming apparatus comprising:

The density conversion means, when forming an image having the gradation, calculates a density difference between a gradation closest to the color of the recording medium and a gradation of the recording medium to a density between other gradations. The image forming apparatus according to claim 1, wherein the difference is larger than the difference .

The image forming apparatus according to claim 1, wherein the image forming apparatus performs image formation based on a monochrome or color electrophotographic process .

A reflectance measurement process for measuring the reflectance of the recording medium being conveyed;
A recording medium brightness index calculation process for calculating a brightness index of the recording medium based on the brightness obtained from the reflectance measured in the reflectance measurement process;
Based on the brightness index of the recording medium calculated in the recording medium brightness index calculation process, the reference brightness index of the recording medium set as a predetermined reference value, and the reference brightness index of the maximum density of the image, the image Maximum density lightness index calculation processing for calculating the maximum density lightness index,
A gradation image determination process for determining the presence or absence of gradation of the image;
When forming an image having a single density according to the presence or absence of the gradation of the image determined by the gradation image determination process, the maximum density of the image calculated by the maximum density lightness index calculation process In the case of forming an image having the gradation while converting to the desired density based on the brightness index of the recording medium, the brightness index of the recording medium calculated by the recording medium brightness index calculation process and the image A lightness index for each gradation is set based on the lightness index of the maximum density, and density conversion processing is performed to convert the lightness corresponding to the set lightness index of each gradation to a density obtained from the lightness. Image forming method.

In the density conversion process, when an image having the gradation is formed, the density difference between the gradation closest to the color of the recording medium and the gradation of the recording medium is calculated as a density between other gradations. The image forming method according to claim 4, wherein the difference is larger than the difference .