JP3770974B2 - Liquid crystal display - Google Patents

Description

この式を用いて考えると、上記の現象は行列の掛け算の中央部から、Ｔ21aとＴ21bの積、Ｔ22aとＴ22bの積、…Ｔ30aとＴ30bの積の順に単位行列となり、理想的には入射偏光と出射偏光が等しくなるものと考えられる。
【００４６】
図１において偏光板７，８を互いの偏光軸が直交するように配置しておけば、表示電圧無印加時には上記のように液晶層１と位相板６の光学伝搬特性は完全に補償し合うので、表示は黒となる。この黒表示は偏光板特性で定まるものであり、非常に良好な視野角特性を持っている。この結果、赤（Ｒ）・緑（Ｇ）・青（Ｂ）のカラーフィルターを形成した３つの画素の混色を用いて色表示を行う場合に、表示電圧無印加状態となっている色が視野角を振っても漏れにくくなり、色相の視野角依存が小さく、コントラストとともに色相変化を考慮した視野角特性も良好なものとなる。反対に、偏光板７，８を互いの偏光軸が平行になるように配置しておけば、表示電圧無印加時に視野角依存性の良好な白表示を行うことができる。
【００４７】
液晶層１に表示電圧を印加すると、正の複屈折媒体である液晶分子１１が立ち上がり、液晶層１と位相板６との光学伝搬特性が上記の補償条件からずれる。このとき、液晶層１と位相板６とのねじれ角およびリターデーションの値や、偏光板７，８の偏光軸と液晶層１や位相板６の主軸方向とのなす角度により表示電圧印加時の表示特性を所望のものに設定することができる。
【００４８】
上記の補償原理は、図４における各薄層の組が完全対称の時に成り立つものではあるが、実際には薄層２１ａと薄層２１ｂにおいて、液晶分子１１の配列方向と負の一軸性光学媒体１２の主軸方向との間のずれが１０度程度であり、かつ、液晶層１と位相板６とのねじれ角の差が２０度以下（すなわち、ねじれ方向の一方を正とした符号を考慮した両者のねじれ角の和が−２０度から＋２０度の範囲内）であれば、偏光板７，８の配置角を若干調整するだけで、実用上十分な視野角特性を得ることができる、つまり、正面から見て、黒の色付きを生じさせることなく、表示することができる。
【００４９】
なお、各光学要素のねじれ角や配置角を定義する場合には、これを液晶層１，位相板６の法線方向から見た場合の角度、即ち界面への正射影で定義するものとし、その傾斜角は考慮しないものとする。
【００５０】
次に、負の一軸性光学媒体１２を有する位相板６を設けることで、液晶層１と位相板６との間で互いの複屈折の影響を打ち消し合う理由を図５を参照して説明する。
【００５１】
図５（ａ）は液晶層１'に正の一軸性光学媒体６２を有する位相板（以下、このような位相板を正の位相板と称す）５２を積層した液晶表示装置のモデル構成であり、図５（ｂ）は同様の液晶層１'に対して負の一軸性光学媒体１２'を有する位相板（以下、このような位相板を負の位相板と称す）６'を積層した本願発明の液晶表示装置のモデル構成を示す断面図である。なお、図５では、説明を簡素化するために、液晶分子１１'、正の一軸性光学媒体６２、および負の一軸性光学媒体１２'を、その主軸を液晶層１'や位相板６'の面方向と平行にして配列している。
【００５２】
液晶分子１１'はその主軸を図中左右方向にして平行配置されている。また、液晶分子１１'は正の一軸性光学媒体としての光学特性を有している。すなわち、軸方向に振動する光に対する屈折率である異常屈折率ｎeLCと、軸方向に対して直角な方向に振動する光に対する屈折率である常屈折率ｎoLCとの間には、ｎeLC＞ｎoLCの関係がある。
【００５３】
正の位相板５２は、正の一軸性光学媒体６２の光軸方向が液晶分子１１'に対して直交するように配置されている。つまり、正の一軸性光学媒体６２はその主軸を紙面に垂直な方向にして平行配列されている。また、正の一軸性光学媒体６２は正の一軸性光学媒体としての光学特性を有している。すなわち、その異常屈折率ｎe1と、常屈折率ｎo1との間にはｎe1＞ｎo1の関係がある。
【００５４】
負の位相板６'は、負の一軸性光学媒体１２'が液晶分子１１'に対して平行となるように配置されている。つまり、負の一軸性光学媒体１２'はその主軸を図中左右方向にして平行配置されている。また、負の一軸性光学媒体１２'は負の一軸性光学媒体としての光学特性を有している。すなわち、その異常屈折率ｎe2、常屈折率ｎo2との間には、ｎe2＜ｎo2の関係がある。
【００５５】
次に、図５（ａ）,（ｂ）の構成に対して、垂直光５６が入射された場合を考察する。垂直光５６は液晶層１'を通過する際に、紙面に垂直な方向に偏波面を持つ偏光成分５６Vと、紙面内向に偏波面をもつ偏光成分５６Hとの間にはある位相差が生じる。５（ａ）の構成と図５（ｂ）の構成のいずれにおいても、液晶層１'のリターデーションと位相板５２，６'のリターデーションとを等しくしておけば、液晶層１'を通過した光には、液晶層１'で生じた位相差を相殺する位相差が位相板５２，６'によって与えられ、これにより全体としては位相差ゼロの状態が作り出される。
【００５６】
このように、垂直光５６では、正の位相板５２であろうと、負の位相板６'であろうと位相板５２，６'を介して出射される光は位相差ゼロとなる。
【００５７】
次に、液晶層１'や位相板５２，６'に対して垂直方向から角度（入射角）αだけ傾斜した方向に伝搬する傾斜光５７が入射された場合を考察する。
【００５８】
傾斜光５７が液晶層１'および位相板５２，６'を通過する距離は、垂直光５６より長くなる。例えば図５(ｂ)に示すように、負の位相板６'の厚みをｄとすると垂直光５７が負の位相板６'を通過する距離はｄ／ｃｏｓαとなる。このことは液晶層１'や正の位相板６２においても同様である。
【００５９】
光学的異方性を鑑みれば、液晶分子１１'や負の一軸性光学媒体１２'は、入射角αが大きくなるほど光学軸に近い方向に光が伝搬することになる。そのため、傾斜光５７の２つの偏光成分５７V，５７Hの間の屈折率異方性はＣＯＳ2αのファクターで減少していくことになる。
【００６０】
一般に、リターデーションは屈折率異方性と伝搬距離との積と見なすことができるので、負の位相板６'では、液晶層１'のリターデーションおよび負の位相板６'のリターデーションは、入射角αの増大とともに、どちらも垂直光５６のリターデーションからｃｏｓαの割合で減少していくことになる。
【００６１】
一方、正の位相板５２では、垂直光５６、傾斜光５７のいずれにおいても、正の一軸性光学媒体６２の長軸と光の伝搬方向とは直交しており、垂直光５６，傾斜光５７の２つの偏光成分５６V，５６H（５７V，５７H）の屈折率異方性は、角度αの変化によって変動することはない。そのため、リターデーションの変動の計算においては、距離のファクターのみが残り、正の位相板５２のリターデーションは、垂直方向のリターデーションから１／ｃｏｓαの割合で増加していくことになる。
【００６２】
したがって、正の位相板５２を有する図５（ａ）の構成に対して、傾斜光５７を入射すれば、液晶層１'のリターデーションが減少する一方で正の位相板５２のリターデーションが増加し、両者の補償関係が崩れていく。
【００６３】
これに対して、負の位相板６'を有する図５（ｂ）の構成に対して、傾斜光５７を入射すれば、液晶層１'のリターデーションと負の位相板６'のリターデーションがともにほぼ等しい量だけ減少するため、両者の補償関係は良好なものに保たれる。
【００６４】
なお、上述した説明では、光の入射角αが垂直方向から図の左右に傾斜した場合に当てはまるものである。これに対して、光の入射角αが紙面の奥行き方向や手前方向に傾斜した場合では、液晶層１'および負の位相板６'は、入射角αが増加しても屈折率異方性が一定であるため、入射角αの増加とともにリターデーションも増加する。これに対して、正の位相板５２では、屈折率異方性は入射角αの増加に伴ってｃｏｓ2αのファクターで減少するため、リターデーションは減少する。
【００６５】
このように、光の入射角αが紙面の奥行き方向や手前方向に傾斜した場合であっても、正の位相板５２を備えた図５（ａ）の構成では、傾斜光５７に対して補償特性が崩れていくのに対して、負の位相板６'を備えた図５（ｂ）の構成では、傾斜光５７に対する補償特性が良好なものに保たれることがわかる。なお、入射角αが、紙面水平方向と紙面垂直方向の中間角度位置にある場合には、そのときのリターデーションは図５（ａ）の状態のリターデーションと図５（ｂ）の状態のリターデーションとの間の中間の値を示す。なお、説明を簡単にするために、上記の説明では、各界面における光の屈折を省略している。
【００６６】
このように、液晶層１'に負の位相板６'を積層すれば、両者のリターデーションの視野角依存性が同―方向に向かい、液晶層１'の単独の場合よりも視野角依存性は緩和されることになる。そのため、負の位相板６'さえ装着すれば、液晶層１'と負の位相板６'とのリターデーションが必ずしも等しくない場合であっても、液晶層１'単独の構成や液晶層１'に正の位相板５２を積層した構成に比べて、液晶表示装置の視野角特性を改善することができる。そして、液晶層１'と負の位相板６'とのリターデーションを互いに同等もしくはそれに準じる状態にすれば、視野角特性はさらに改善されることになる。
【００６７】
上述した説明では、説明を簡単にするために液晶層１'と各位相板５２，６'との双方にねじれ構造がないものとしているが、この原理はねじれ構造のある場合にも当てはまる。特に、図４等に示すように液晶層１と位相板６とをいくつかの薄層に分割して各組み合わせでの光学的補償を考えることができる場合には、各薄層はねじれ角が小さくかつ平行配列に近似したものと考えることができるため、上述した説明をそれぞれの薄層の組み合わせをそのまま適用することができる。
【００６８】
ところで、実際のＳＴＮ型液晶表示装置は、デューティー駆動される場合がほとんどである。この場合には、オフ状態でも液晶層１には若干の電圧（オフ電圧）が印加されており、液晶分子１１がやや立ち上がった状態にある。そのため、正面方向から見た液晶層１１のリターデーションは表示電圧の全く印加されていない状態よりやや小さくなっている。そのため、位相板６のリターデーションを液晶層１のリターデーションよりやや小さめに設定することが、より良好な正面方向の表示特性を得る意味で望ましい。
【００６９】
図６は、位相板６のリターデーションＲnと液晶層１のリターデーションＲLCとの間の比Ｒn／ＲLCと、表示のコントラストとの間の関係に関する実験結果を示すものである。この実験の条件は次の通りである。すなわち、液晶表示装置の駆動デューティー比は１／３００、液晶層１のリターデーションは８４０ｎｍ、液晶層１のねじれ角は左回り２５０度、位相板６のねじれ角は右回り２５０度である。また、偏光板８の偏光軸方向と液晶層１下面の液晶分子の配列方向のなす角度は４５度、偏光板７の偏光軸と偏光板８の偏光軸とは直交配置である。
【００７０】
この実験結果から明らかなように、コントラスト２０以上という比較的高いコントラストを得るためには、Ｒn／ＲLCを、０.８５〜０.９６の間に設定するのが望ましい。さらには、コントラスト４０以上という高コントラストを得るためには、Ｒn／ＲLCを０.８８〜０.９５の間に設定するのが望ましい。
【００７１】
図７は、上記の比（Ｒn／ＲLC）が、０.９２である場合のコントラストの視野角依存性を示したものである。図７には、全方位角について倒れ角８０度までの等コントラスト曲線が描かれている。また、図８は、同様の条件で、正の屈折率異方性を有する一軸性光学媒体をねじれ配置した位相板を、その液晶層側界面において、その光学媒体の主軸が液晶分子の配向方向と直交するように配置した場合のコントラストの視野角依存性を図８に示す。
【００７２】
これらの図において周辺部に右上がりの斜線で示す領域イは、コントラストが１以下で表示が反転していることを示しているが、負の屈折率異方性をもつ一軸性光学媒体を設けた図７の特性は正の屈折率異方性をもつ一軸性光学媒体を設けた図８の特性に比べて、この表示反転領域イが大幅に減少している。また、中央部に左上がりの斜線で示すコントラスト２０以上の領域ロは、図７の方が広くなっている。すなわち、負の屈折率異方性を有し、かつ液晶層１とはねじれ方向が逆となった一軸性光学媒体の使用により、視野角特性が大幅に改善されることが分かる。
【００７３】
デューティー駆動を行うためには、輝度−電圧特性が急峻である必要があるが、この急峻性を得るために、液晶層１における液晶分子のねじれ角は２００度以上であることが望ましく、２４０度以上であればさらに良好な表示特性が得られる。一方、ねじれ角が大きすぎる場合には配向不良現象が生じやすくなるので、ねじれ角は３００度以下、さらに望ましくは２７０度以下に設定するのがよい。
【００７４】
また、偏光板８の偏光軸と、液晶層１の偏光板側界面１ａにおける液晶分子の配向方向との間に形成される角度は、上記と同様に特性の急峻性を得る意味で、１５度〜７５度の範囲、さらに望ましくは３０度〜６０度の範囲に設定するのが望ましい。偏光板７の偏光軸方向は、液晶層１と位相板６のねじれ角が等しい場合には、偏光板８の偏光軸方向とほぼ直交させればよく、両者のねじれ角に差がある場合は、その分だけ回転させればよいが、上記の各方向から数度から十数度の範囲で調整すれば、さらに良好な表示特性を得ることができる。なお、白表示の視野角特性を良好にしたい場合には、偏光板７，８の偏光軸を平行方向、あるいはそれにねじれ角の差を加えた方向から数度あるいは十数度の範囲で調整するのが望ましい。
【００７５】
液晶層１のリターデーションは、小さすぎる場合にはオン輝度が不足し、大きすぎる場合にはオン表示が黄色っぽく着色する。そこで、白黒表示やＲＧＢカラーフィルタによる３色混合のカラー表示を行う場合には、液晶層１のリターデーションは７００〜９５０ｎｍの間に設定するのが良く、７８０〜９００ｎｍの間であればさらに良好な表示特性を得ることができる。特に、ねじれ角が２５０度前後の場合には、液晶層１のリターデーションを８４０ｎｍ付近に設定すれば、非常に良好な表示特性を得ることができる。
第２の実施の形態
本実施の形態の液晶表示装置は、基本的には第１の実施の形態の液晶表示装置と同様の構成をもっている。第１の実施の形態との主な違いは、位相板の構成にある。第１の実施の形態では、デューティー駆動時の正面からの表示特性を良好なものにするために、位相板６のリターデーションを液晶層１のリターデーションよりやや小さめに設定した。これに対して、本実施の形態の位相板１４は、図９に示すように負の１軸性光学媒体１５の主軸を位相板１４の面方向から図中右上がりに角度θだけチルト傾斜させたうえで、ねじれ配列させたことにより、デューティー駆動時の正面特性を良好なものにしている。図９では、負の一軸性光学媒体１５の主軸チルト角θは、負の一軸性光学媒体１５の位相板１４の深さ方向位置ｚに係わらず一定であるとしているが、これは若干の分布をもっていても構わない。
【００７６】
位相板１４における深さ方向の位置ｚでの主軸のチルト角がθｚであるとし、固有複屈折をΔｎ、位相板の厚みをｄとする。この場合には、位相板１４の正面方向からの等価的なリターデーションＲeffは、次の（２）式で表される。
【００７７】
【数２】

位相板１４の正面方向からの等価的なリターデーションＲeffと、液晶層１の表示電圧無印加時のリターデーションＲLCとの間の比Ｒeff／ＲLCを、第１の実施形態と同様に、０.８５〜０.９６の間、さらに望ましくは０.８８〜０.９５の間に設定すれば良好な表示特性を得ることができる。
【００７８】
なお、負の一軸性光学媒体１５に上述したチルト構造がない場合には、リターデーションＲeffの値は通常のリターデーションと一致する。チルト角θを決定することが困難でリターデーションＲeffの値を上記した(２）式から求めることができない場合には、位相板１４に偏光を入射させて出射偏光の状態を観察し、これをシミュレーションによって求めた理論的な出射偏光状態と比較すれば、比較的容易に等価リターデーションの値を求めることができる。
【００７９】
チルト角θの方向は、位相板１４の液晶側界面での負の一軸性光学媒体１５の主軸と、液晶層１の位相板側界面１ａでの液晶分子の主軸とが同一方向に向くように、すなわち、図４において薄層２１ａと薄層２１ｂとの主軸が同一方向に傾くように設定するのが、視野角を拡大するうえで望ましい。液晶層１のねじれ角およびリターデーションの値、さらに偏光板７，８の配置については第１の実施形態と同様に設定すればよい。
【００８０】
本実施の形態は、位相板１４における負の１軸性媒体１５の主軸を位相板１４の面方向からやや立ち上がった方向に傾けている（チルト傾斜している）ので、第１の実施の形態に比べて、オフ表示状態であっても若干のオフ電圧が印加されるためやや液晶分子が立ち上がった液晶層１と位相板１４との光学的な整合性がよくなり、視野角に対する補償特性が向上する。
第３の実施の形態
本実施の形態の液晶表示装置は、基本的には第１，第２の実施の形態の液晶表示装置と同様の構成をもっている。第１，第２の実施形態との主な違いは、位相板の構成にある。本実施の形態では、第２の実施の形態の考えをさらに推し進めて、次のような位相板１６を有している。すなわち、位相板１６は、液晶分子１１のチルト具合に対応してそのチルト角が深さ方向の分布を持った負の一軸性光学媒体１７を有している。
【００８１】
単純マトリクス型の液晶表示装置では、その構造上、オフ表示状態でも若干の電圧（以下、この電圧をオフ電圧と称す）が液晶層１に印加される。液晶層１にこのようなオフ電圧が印加された状態では、液晶層１と透明基板４，５との界面において、いわゆるアンカリング現象のため液晶分子１１は初期配列状態からほとんど動かないものの、液晶層１の厚み方向中央部付近では電圧変化に伴って液晶分子１１の動きが生じる。
【００８２】
オフ電圧を印加した場合に、液晶層１の深さ方向の位置ｚに応じた液晶分子の配列方向は、シミュレーションによって求めることができる。図１０は、オフ電圧印加時における液晶厚み方向の各位置ｚに対する液晶分子のチルト角θ・ツイスト（ねじれ）角φを計算した結果を示している。なお、この計算ではプレチルト角を６度としている。
【００８３】
本実施の形態では、この計算結果に対応させて、位相板１６が有する負の一軸性光学媒体１７の主軸のチルト傾斜を、図１１に示すように分布させた構成をとっている。すなわち、負の一軸性光学媒体１７のツイスト角を、液晶分子のツイスト角とは逆向きにしたうえで、負の一軸性光学媒体１７のチルト角の厚み方向の分布を、オフ電圧印加時（表示電圧無印加時）における液晶分子のチルト角の厚み方向の分布に一致させている。
【００８４】
本実施の形態における液晶分子１１と位相板１６の負の一軸性光学媒体１７との配列関係は図１２に示すものとなる。図１２は図４に準じた図であって、同等の部分には、同一の符号を付している。
【００８５】
図１２においても、図４と同様、薄層２１ａと薄層２１ｂ、薄層２２ａと薄層２２ｂ、…薄層３０ａと薄層３０ｂの各組み合わせにおける液晶分子１１の主軸と負の一軸性光学媒体１７の主軸とが互いに平行となっている。そのため、第１の実施の形態での説明と同様の原理で、薄層の各組み合わせが、さらに良好に相互の光学的伝搬特性を相殺しあい、広い視野角特性を得ることができる。
【００８６】
本実施の形態は、液晶層１にオフ電圧が印加された場合の光学特性を補償するように位相板１６の負の一軸性光学媒体が配列しているので、オフ表示状態でもオフ電圧が印加される構造においては、第１および第２の実施の形態に比べて、さらに良好な視野角特性を得ることができる。
【００８７】
なお、上記の第１，第２，第３の実施の形態において、位相板６，１４，１６は液晶層１から見て前面側にあるものとし、図１において偏光板８の背後側にバックライト９を配置した。しかしながら、偏光板７の背後にバックライト９を配置し、偏光板８の側から表示を観察するようにしても構わない。この場合も、上記の説明と同様にして良好な視野角特性の表示を行うことができる。
第４の実施の形態
図１３は、本発明の第４の実施の形態の液晶表示装置の構成を示すものである。本実施の形態は、負の一軸性光学媒体を有する位相板を第１の位相板６Ａと第２の位相板６Ｂとの２枚に分割し、これら位相板６Ａ，６Ｂを液晶層１の両側に配置したことに特徴がある。
【００８８】
本実施の形態においては、液晶層１の第１の位相板側界面１ｂにおける液晶分子（図示省略）と、第１の位相板６Ａの液晶層側界面６ｂにおける負の一軸性光学媒体（図示省略）の主軸とがなす角度は理論的には平行であるのが良い。同様に、液晶層１の第２の位相板側界面１ｃにおける液晶分子（図示省略）と、第２の位相板６Ｂの液晶層側界面６ｃにおける負の一軸性光学媒体（図示省略）の主軸とがなす角度は理論的には平行であるのが良い。しかしながら、第１の実施の形態で説明したのと同様に、実用的には上述した各角度が１０度以下であれば良好な視野角特性を得ることができる。
【００８９】
光学補償の原理は、第１の実施形態で説明したように、液晶層１と第１，第２の位相板６Ａ，６Ｂとを薄層に分割し、液晶層１と各位相板６Ａ，６Ｂとの薄層の組み合わせが光学特性を順次キャンセルするものと考えることができる。
【００９０】
さらに、本実施の形態の構成では、例えば、第１，第２の位相板６Ａ，６Ｂのリターデーションを等しく設定した場合には、液晶層１の上半分の特性を上側に位置する第１の位相板６Ａが補償し、液晶層１の下半分の特性を下側に位置する第２の位相板６Ｂが補償することになる。これにより、各位相板６Ａ，６Ｂが補償する液晶層１の厚みは第１の実施の形態の構成の半分で済むことになる。そのため、薄層の組み合わせで光学補償で考えた場合、補償特性にミスマッチがあった場合、すなわち、液晶層１と位相板６Ａ，６Ｂとの間で、対応する薄層の光学特性の整合性に不一致があった場合でも、その影響が蓄積されにくく、結果的に良好な視野角特性を示すことになる。また、液晶層１の上下それぞれに第１の位相板６Ａないし第２の位相板６Ｂがあるので構成の対称性が増し、視野角特性がより対称なものになるという利点もある。なお、第１，第２の位相板６Ａ，６Ｂどうしのリターデーションは必ずしも等しくなくても、ここで説明した効果は発揮される。
【００９１】
第１の実施の形態において、位相板６のリターデーションＲnと液晶層１のリターデーションＲLCとの比Ｒn／ＲLCは、０.８５〜０.９６の間に設定するのが望ましく、０.８８〜０.９５の間に設定するのがさらに望ましいと述べたが、本実施の形態においては次の通りである。すなわち、第１の位相板６ＡのリターデーションＲn1と第２の位相板６ＢのリターデーションＲn2との和（Ｒn1＋Ｒn2）と液晶層１のリターデーションＲLCとの比（Ｒn1＋Ｒn2）／ＲLCが０.８５〜０.９６の間であるのが望ましく、０.８８〜０.９５の間に設定するのがさらに望ましい。
【００９２】
また、ねじれ角については、液晶層１と第１，第２の位相板６Ａ，６Ｂのねじれ角を、一方のねじれ方向を正とした符号を考慮して加え合わせた総和が−２０度から＋２０度の範囲内にあれば、実用上十分な視野角特性を得ることができる。
【００９３】
偏光板の配置角については、偏光板８の偏光軸と、第２の位相板６Ｂの偏光板８側界面６ｄにおける負の一軸性光学媒体（図示省略）の主軸との間に形成される角度を、望ましくは１５度〜７５度の範囲、さらに望ましくは３０度〜６０度の範囲に設定すれば、輝度−電圧特性が急峻で、マルチプレックス駆動に適した液晶特性を得ることができる。
【００９４】
図１４は、第１の実施の形態で用いた位相板６をねじれ角が１２５度でリターデーションが等しい２枚の位相板６Ａ，６Ｂに分割し、液晶層１の両側に配置して構成した第４の実施の形態の液晶表示装置のコントラストの視野角依存性を示している。
【００９５】
この図から明らかなように、第１の実施の形態の液晶表示装置の視野角依存性（図７参照）に比べて、本実施の形態の液晶表示装置の視野角依存性（図１４参照）は、コントラスト５０以上の傾域が増大し、かつ視野角特性の左右対称性が向上している。例えば、図中、上方においてはコントラスト５０以上の領域（図７，図１４で符号ハで表示）が倒れ角にして約１０度拡大し、図中、右上と右下方向ではコントラスト２０以上の領域ロが倒れ角にして約１０度拡大している。この結果、本実施の形態の液晶表示装置では、左右対称で視野角の広い非常に見やすい表示を得ることができる。
【００９６】
本実施の形態の液晶表示装置を、黒表示の視野角依存性が良好なＮＢ表示形態にしたい場合には、偏光板７の偏光軸は偏光板８の偏光軸と直交する方向から上記のねじれ角の総和の分だけすらした方向に配置すればよい。このとき、上記の方向から数度から十数度の範囲で調整すれば、なお良好な表示特性を得ることができる。なお、白表示の視野角特性が良好なＮＷ表示形態にしたい場合には、偏光板７の偏光軸は偏光板８の偏光軸と平行な方向を基準として同様に考えればよい。
【００９７】
本実施の形態のように、液晶層１の両側に負の位相板（負の屈折率異方性をもつ一軸性光学媒体を有している）６Ａ，６Ｂを配置した液晶表示装置においても、第２の実施の形態に示したように、各位相板６Ａ，６Ｂの負の一軸性光学媒体の主軸を傾けて視野角特性をさらに良好なものにすることもできる。同様に、第３の実施の形態に示したように、各位相板６Ａ，６Ｂの負の一軸性光学媒体を液晶分子の配列状態に応じて配列させて、視野角特性をさらに良好なものにすることもできる。
第５の実施の形態
図１５に本発明の第５の実施の形態である液晶表示装置の構成を示す。図において、第１の実施の形態と同一ないし同等の部分には同一の符号を付し、その説明は省略する。
【００９８】
本実施の形態の液晶表示装置の特徴は、液晶層１と負の屈折率異方性をもつ一軸性光学媒体よりなる位相板（負の位相板）６との間に、正の屈折率異方性をもつ一軸性光学媒体１９を備えた位相板（正位相板：請求項の第２の位相板に相当する）１８を更に設けたことにある。
【００９９】
図１６は、液晶層１と２つの位相板１８，６における正負の一軸性光学媒体１９，１２の配列を模式的に示すものである。本図では、透明基板４，５等の部品は記載を省略している。位相板１０は、正の屈折率異方性をもった正の一軸性光学媒体１９を有しており、正の一軸性光学媒体１９は液晶層１と同じ方向にねじれ配列している。位相板１８の液晶層側界面１８ａでの正の一軸性光学媒体１９の主軸方向は、液晶層１の位相板１８側界面１ｄでの液晶分子１１の主軸方向とほぼ一致している。また、正の一軸性光学媒体１９のねじれ方向は液晶分子１１のねじれ方向と同一方向であり、液晶層１における液晶分子１１のねじれ角と位相板１８における正の一軸性光学媒体１９のねじれ角を合計するとほぼ３６０度となっている。
【０１００】
位相板６は、第１の実施の形態と同様に、負の屈折率異方性をもち、かつ、液晶層１とは逆の方向にねじれて配列した負の一軸性光学媒体１２を有している。
【０１０１】
本実施の形態においても、第１の実施の形態と同様の原理で、液晶層１と位相板１８によって生じる視野角依存を位相板６が補償するので、非常に視野角の広い液晶表示装置を得ることができる。これは、第１の実施の形態において説明したように、位相板６，１８と液晶層１とをいくつかの薄層に分割し、正負の屈折率異方性をもつ薄層の組み合わせが互いに光学特性を補償するものと考えることができる。理想的状態では、全方位で完全に光学補償が行われるが、現実には屈折率の値や、液晶分子１１および各一軸性光学媒体１２，１９の主軸の向きが必ずしも理想的なものではないため、各薄層の組み合わせにおける軸方向（位相板６，１８，液晶層１の法線方向）近傍に光の進行方向がある場合には補償特性が良好となり、それから光の進行方向がはずれるに従って補償が不完全になることが多い。
【０１０２】
しかしながら、本実施の形態は第１の実施形態に比べて、正の一軸性光学媒体１９を有する位相板１８を設けているので、上述したような各薄層の組み合わせにおける一軸性光学媒体や液晶分子の主軸の方向が３６０度すべての方向に対して存在し、視野角補償における軸方向依存性があらゆる方向に分散され、さらに良好な視野角特性を得ることができる。
【０１０３】
以上の説明を別の視点からみると次にようになる。すなわち、正の一軸性光学媒体１９を有する位相板１８は液晶表示の視野角特性における方位角依存を緩和あるいは平均化し、負の一軸性光学媒体を有する位相板６はこの視野角依存を補償する働きを有するものと考えられる。
【０１０４】
この補償は、第１の実施の形態と同様に表示電圧無印加状態（オフ状態）に対して特に有効であるため、偏光板７，８の偏光軸をほぼ直交に配置（ＮＢ表示）にすれば、非常に良好な視野角の黒表示を得ることができ、偏光板７，８の偏光軸をほぼ平行に配置（ＮＷ表示）にすれば、非常に良好な視野角の白表示を得ることができる。
【０１０５】
本実施の形態の構成では、液晶分子１１や正負の一軸性光学媒体１２，１８のねじれ角の関係が次の示す（３）式の範囲にあることが望ましく、さらには、（４）式の範囲にあることがさらに望ましい。
【０１０６】
-30°≦ΩLC＋Ωp＋Ωn≦30°…（３）
-10°≦ΩLC＋Ωp＋Ωn≦10°…（４）
ΩLC：液晶分子１１のねじれ角
Ωp：正の一軸性光学媒体１９のねじれ角
Ωn：負の一軸性光学媒体１２のねじれ角
なお、ねじれ方向には符号を付けており、左ねじれを正、右ねじれを負としている。
【０１０７】
本実施の形態においては、液晶層１のねじれ角、偏光板８の偏光軸と液晶分子１１との間に形成される角度、および、偏光板７の偏光軸と偏光板８の偏光軸との間に形成される角度については、第１の実施の形態と同様の範囲に設定するのが望ましい。
【０１０８】
液晶層１と各位相板６，１８との近接面において、液晶分子１１や正負の一軸性光学媒体１９，１２の配向方向や主軸方向が相互になす角度、すなわち、図１５における液晶層１の位相板側界面１ｄにおける液晶分子１１の配向方向と位相板１８の液晶層側界面１８ａにおける正の一軸性光学媒体１９の主軸方向との間に形成される角度、および位相板１８の位相板６側界面１８ｂにおける正の一軸性光学媒体１９の主軸方向と位相板６の液晶層側界面６ｅにおける負の一軸性光学媒体１２の主軸方向との間に形成される角度について説明する。
【０１０９】
すなわち、液晶層１に電圧が全く印加されていない状態を考えると、これらの角度は理論的には平行であるのが理想状態である。しかしながら、実際にはこれらの角度がともに１０度以内であれば実用的に十分良好な表示電圧無印加状態での視野角特性が得られるため、表示電圧印加状態での輝度特性や色相といった他の表示特性をも考慮して表示電圧無印加状態で上述した範囲内になるように、これらの角度を調整すれば、総合的な表示特性をさらに良好なものとすることができる。
【０１１０】
正の一軸性光学媒体１９を有する位相板１８は液晶層１の延長とみなされる構造をもつのが望ましい。したがって、位相板１８ののリターデーションＲpは次に示す（５）式によって算出される値Ｒpoを基本として考えればよい。
【０１１１】
Ｒpo ＝ ＲLC×Ωp／ΩLC…（５）
ＲLC：液晶層１のリターデーション
Ωp：正の一軸性光学媒体１９のねじれ角
ΩLC：液晶分子１１のねじれ角
しかしながら、液晶表示装置がデューティー駆動されることを考慮して、位相板１８のリターデーションをこれより１０％程度小さい値に設定したり、正の一軸性光学媒体１９を傾斜配列させたりすれば、さらに液晶層１との光学的な連続性が良好になる。なお、正の一軸性光学媒体１９を傾斜配列する場合には、傾斜配列の方向は液晶分子１１の傾斜方向と一致させるのが望ましい。
【０１１２】
傾斜構造をも考慮した位相板１８の等価的なリターデーションＲpeffは、第１の実施の形態と同様に考えて、次に示す（６）式によって算出される値となる。
【０１１３】
【数３】

Δｎp：正の一軸性光学媒体１９の固有複屈折
なお、傾斜配列していない場合には、位相板１８の等価的なリターデーションＲpeffは上述した（５）式で算定されるＲpoに基づいて選定されるリターデーションＲpに等しくなる。
【０１１４】
本実施の形態の構成においては、負の一軸性光学媒体１２を有する位相板６のリターデーションのうち一部分が正の一軸性光学媒体１９を有する位相板１８の光学特性を補償し、位相板６のリターデーションの残りの部分が液晶層１の光学特性を補償するものと考えられる。したがって、位相板６のリターデーションのうち液晶層１の光学特性を補償する部分は、Ｒn−Ｒpeffとなる。
【０１１５】
第１の実施の形態で説明したように、液晶層１はデューティー駆動されるので、液晶層１の光学特性を補償する部分であるＲn−Ｒpeffと液晶層１のリターデーションとの間に第１の実施の形態で説明した関係が成り立つのが望ましい。すなわち、良好な視野角特性を実現するためには次の示す（７）式の関係が成り立つことが望ましく、さらには、次に示す（８）式の関係が成り立てばより良好な視野角特性を得ることができる。
【０１１６】
０.８５≦（Ｒn−Ｒpeff）／ＲLC≦０.９６…（７）
０.８８≦（Ｒn−Ｒpeff）／ＲLC≦０.９５…（８）
図１７は本実施の形態の液晶表示装置における、コントラストの視野角依存性を示すものである。図７に示す第１の実施の形態の視野角特性に比べて、表示反転領域にはほとんど差がないものの、コントラスト２０以上の領域ロやコントラスト５０以上の領域ハが図中左下方向に広がり、視野角特性が向上していることが分かる。
第５の実施の形態の変形例１
第５の実施の形態の液晶表示装置においては、第２の実施の形態と同様に、負の一軸性光学媒体を有する位相板６として、主軸がチルト配列した負の一軸性光学媒体１５を有する位相板１４と同様の位相板を用いてもよい。以下、このような位相板を位相板６(14)と称す。
【０１１７】
そうすれば、第２の実施の形態での説明と同様の原理で、デューティー駆動時の正面特性は良好なものとなる。すなわち、位相板６(14)の正面方向からの等価的なリターデーションＲneffは、前述した（２）式で求めるＲeffの値に等しくなる。また、第５の実施の形態の上述した説明では、リターデーションＲnの値を前述した（７）式および（８）式で示す範囲に設定するのが望ましいと説明したが、位相板６(14)を用いれば、リターデーションＲnをリターデーションＲneffに置き換えて考えればよい。
【０１１８】
なお、位相板６(14)が有する負の一軸性光学媒体のチルト方向は次のように設定すればよい。すなわち、液晶層１のチルト構造を正の位相板内に仮想的に延長する。そして、この仮想チルト構造に対して、位相板６(14)における負の一軸性光学媒体の主軸のチルト方向が連続するように、位相板６(14)が有する負の一軸性光学媒体のチルト方向を設定するのが望ましい。
【０１１９】
位相板６(14)を設けた場合には、位相板６(14)が有する負の一軸性光学媒体の主軸を位相板６(14)の面方向からやや立ち上がった方向に傾けているので、通常の位相板６を有する第５の実施の形態の構造に比べて、オフ電圧のためにやや液晶分子１１が立ち上がった液晶層１と位相板６(14)との光学的な整合性がよくなり、視野角に対する補償特性がさらに向上する。
第５の実施の形態の変形例２
第５の実施の形態の液晶表示装置においては、負の１軸性光学媒体１２を有する位相板６として、第３の実施の形態で示した負の一軸性光学媒体１７を有する位相板１６と同様の位相板を用いてもよい。以下、このような位相板を位相板６(16)と称し、この位相板６(16)が有する負の一軸性光学媒体を負の一軸性光学媒体１２(17)と称す。
【０１２０】
負の一軸性光学媒体１２(17)は、次のような構成を備えている。すなわち、負の一軸性光学媒体１２(17)のチルト角は位相板６(16)の深さ方向に沿った所定の分布を持っている。この所定の分布は液晶層１の液晶分子１１のチルト具合および正の位相板１８の正の一軸性光学媒体１９の主軸のチルト具合に対応している。
【０１２１】
液晶層１にオフ電圧を印加した場合の液晶分子１１の配列分布は第３の実施の形態と同様にしてシミュレーションすることによって求めることができる。本実施の形態では、これに加えて正の一軸性光学媒体１９の主軸も考慮に入れ、これらに対応して負の一軸性光学媒体１２(17)の主軸のチルト角を位相板６(16)の厚み方向に分布させた構成となっている。
【０１２２】
この変形例２においては、負の一軸性光学媒体１２(17)の主軸方向のチルト角を上述したように分布させることによって、第３の実施の形態で説明したのと同様に、液晶層１にオフ電圧が印加された場合の光学特性がより忠実に補償される。したがって、第５の実施の形態に比べて、さらに良好な視野角特性が得られる。
第５の実施の形態の変形例３
上述した第５の実施の形態およびその変形例１，２に、正負の位相板１８，６のいずれか一方または双方を２枚の位相板に分割し、液晶層１の上下に１枚づつ配置したところ、さらに良好な表示特性を得ることができる。
【０１２３】
このように構成することで良好な表示特性が得られる理由は次の通りである。すなわち、正の一軸性光学媒体１９を有する位相板１８を分割すると、液晶層１から見て正の位相板１８が対称的に配置されることになる。また、負の一軸性光学媒体１２を有する位相板６を分割すると、対称性が増すことに加えて、第４の実施の形態で説明したように正負の複屈折特性がキャンセルすべき深さ方向の距離が短くて済むことになる。したがって、どちらかといえば負の位相板６を分割する方が、より大きな視野角特性向上効果が得られる。
【０１２４】
なお、第５の実施の形態およびその変形例においては、液晶層１，位相板１８，６が光源側から見てこの順に並んでいるものとした。しかしながら、液晶層１，位相板１８，６の積層順についてはこのような限定はなく、２枚の偏光板７，８の間に挟まれている限りこれらの層を別の順序で積層しても構わないし、偏光板７の側にバックライトを配置し偏光板８の側から表示を観察するような構成にしても構わない。この場合も、第５の実施の形態で説明したのと同様の原理で良好な視野角特性が得られる。
【０１２５】
また、液晶層１と各位相板６，１８の互いの近接面における液晶分子や光学媒体の主軸が相互になす角度は、すべての近接面で１０度以内にするのが望ましい。すなわち、図１５における液晶層１の位相板側界面１ｄにおける液晶分子１１の配列方向と位相板１８の液晶層側界面１８ａにおける正の一軸性光学媒体１９の主軸方向との間に形成される角度、および位相板１８の位相板６側界面１８ｂにおける正の一軸性光学媒体１９の主軸方向と位相板６の液晶層側界面６ｅにおける負の一軸性光学媒体１２の主軸方向との間に形成される角度は、すべて１０度以内にするのが望ましい。
【０１２６】
さらには、第５の実施の形態およびその変形例において、位相板１８，６の負の一軸性光学媒体の主軸がチルト角をもつ場合には、位相板１８と液晶層１との間、および位相板６と液晶層１との間で互いに近接し合う界面（界面１８ａと界面１ｄ、および界面１８ｂと界面６ｅ）においてチルト角が連続に近くなるように、位相板１８，６の各界面における正負の一軸性光学媒体の主軸の方向およびプレチルト角を設定するのが望ましい。
【０１２７】
さらにまた、第５の実施の形態およびその変形例において、偏光板８の偏光軸と、液晶層１の界面１ｄにおける液晶分子の配向方向との間に形成される角度は、１５度〜７５度の範囲、さらに望ましくは３０度〜６０度の範囲に設定すれば、輝度−電圧特性が急峻で、マルチプレックス駆動に適した液晶特性を得ることができる。
【０１２８】
また、第５の実施の形態およびその変形例において、表示電圧無印加状態（オフ状態）を黒表示（ＮＢ表示）として黒表示の視野角依存性を良好なものにしたい場合には、偏光板７の偏光軸方向を、偏光板８の偏光軸方向と直交する方向からねじれ角の総和の分だけずらして配置すればよいが、直交方向から数度から十数度の範囲で調整すれば、なお良好な表示特性を得ることができる。表示電圧無印加時（オフ時）を白表示（ＮＷ表示）として白状態の視野角特性を良好にしたい場合には、偏光板７，８の偏光軸を平行方向を基準として上記と同様に考えればよい。
【０１２９】
以上は、本発明をＳＴＮ型液晶表示装置に実施した形態の説明であったが、次に、本発明をＴＮ型液晶表示装置に実施した形態を説明する。
第６の実施の形態
本実施の形態の液晶表示装置の構成は基本的には図１に示すものと同じであるので、図１を参照してその構造を説明する。この液晶表示装置は、ねじれ角９０度のＴＮ型液晶からなる液晶４０と、負の一軸性光学媒体（図示省略）を液晶層４０とは逆の方向に９０度ねじれ配列させた位相板４１とを備えている。液晶層４０と位相板４１とのリターデーションはほぼ等しく、液晶層１の位相板側界面４０ａにおける液晶分子（図示省略）の配列方向と、位相板４１の液晶層側界面４１ａにおける負の一軸性光学媒体の主軸方向とはほぼ平行となっている。なお、図示していないが、透明基板４２には電圧スイッチング素子としてのＴＦＴが形成されている。このＴＦＴは、ＭＩＭなどのダイオード素子で代用することもできる。
【０１３０】
本実施の形態の液晶表示装置においては、ＴＮモードの動作を行わせるために、偏光板８の偏光軸を、液晶層４０の偏光板８側界面４０ｂにおける液晶分子の配列方向に対して平行あるいは直交させて配置している。偏光板７の偏光軸は、偏光板８の偏光軸と直交するように配置され、このようにして偏光板８，７を配置することでＮＢ表示を行っている。
【０１３１】
この液晶表示装置においては、液晶層４０のリターデーションが小さすぎる場合には表示電圧印加時の明るさが十分でなくなる一方、液晶層４０のリターデーションが大きすぎる場合には視野角補償特性が低下したり、液晶層４０の厚み（セル厚）が厚くなって応答速度が低下したりする。このことから、液晶層４０のリターデーションは３００ｎｍ以上に設定するのが望ましく、４００〜５００ｎｍ付近に設定するのがさらに望ましい。
【０１３２】
本実施の形態においても、光学補償の原理は第１の実施の形態で説明したのと同様の原理であり、表示電圧無印加状態で非常に良好な視野角特性の黒表示を行うことができる。
【０１３３】
ところで、一般に、透明基板にスイッチング素子を設けたアクティブマトリクス型の液晶表示装置では、コントラストが高く中間調特性が良好であるので、ＲＧＢ３つの色画素の混色により、フルカラー画像を表示するディスプレイとして用いられることが多い。この場合、黒表示が視野角によって変化しないことは、表示コントラストから見た視野角範囲を拡大するとともに、視野角による表示色相の変化をなくす意味で特に重要である。ＮＷ表示に基づく従来の液晶表示装置では、印加する表示電圧が不足すると黒表示特性が劣化するため極端に視野角特性が悪くなる。これに対して、本実施の形態の液晶表示装置においては、良好な黒表示が表示電圧ゼロの状態で実現されているので、駆動ＩＣの耐圧や駆動信号の制約により液晶層４０に十分な電圧が印加できない場合でも、白輝度がわずかに低下するのみで、コントラストや色相の視野角依存は非常に良好なものに保たれる。これは、携帯用機器などで電源電圧が十分に取れない場合や、消費電力を低減したい場合、さらには搭載機器からの電磁波の輻射を低減したい場合に、視野角特性を大幅に低下させなくてよいという利点につながる。
【０１３４】
表１は本発明の液晶表示装置と、２種類のＮＷ表示の液晶表示装置（従来例）とについて、駆動電圧５ボルトおよび３．３ボルトで駆動させた場合の表示特性を比較した実験結果である。
【０１３５】
【表１】

すなわち、本実施の形態のＮＢ表示の液晶表示装置を、３.３ボルトと５ボルトとの２種類の信号電圧で駆動させた場合を考える。この場合、３.３ボルト駆動では、５ボルト駆動に比べて白表示の透過率が１０％程度低下するものの、正面から見た場合の黒表示の状態は１００対１以上の非常に良好なコントラストを得ることができ、５ボルト駆動と同等の結果が得られる。一方、ＮＷ表示では、３．３ボルト駆動すると、両方式とも十分なコントラストを得ることができない。
【０１３６】
また、本実施の形態の液晶表示装置を３.３ボルトの信号電圧で駆動した場合の視野角特性は、本発明の特徴となる位相板を設けたＮＢ表示の液晶表示装置を５ボルトの信号電圧で駆動した場合に比べてほとんど遜色がなく、本発明の特徴である位相板を有しない従来のＮＷ型の液晶表示装置を５ボルトの信号電圧で駆動した場合や従来型の位相板付きのＮＷ表示の液晶表示装置を３．３ボルト駆動した場合に比べるとはるかに良好なものとなる。
【０１３７】
また、本実施の形態の液晶表示装置では、ＮＷ表示の５ボルト駆動と同様に低粘性の液晶材料が使用できるため、応答へ速度も５Ｏｍｓｅｃ以下と十分良好となる。
【０１３８】
このことから、本実施の形態の液晶表示装置は、携帯型コンピュータをはじめとする３.３ボルトロジックの各種機器に、液晶表示装置用の昇圧回路を付加することなく組み込めるという特長を有し、機器のコンパクト化や低価格化に貢献することができる。また液晶表示装置での消費電力も約半分に低減され、携帯機器の電池寿命を伸ばしたり、バッテリー部分を軽量化したりすることができる。
【０１３９】
なお、上述した説明では、液晶層と位相板とは、リターデーションとねじれ角が等しく、液晶層１の位相板側界面４０ａにおける液晶分子（図示省略）の配列方向と、位相板４１の液晶層側界面４１ａにおける負の一軸性光学媒体の主軸方向とはほぼ平行となっているとしたが、本実施の形態はこのようなものに限定されるものではない。
【０１４０】
すなわち、液晶層１の位相板側界面４０ａにおける液晶分子（図示省略）の配列方向と、位相板４１の液晶層側界面４１ａにおける負の一軸性光学媒体の主軸方向との間に形成される角度が１０度以下、さらに望ましくは５度以下であり、一方のねじれ方向を正とした符号を考慮した、液晶層４０と位相板４１とのねじれ角の総和が−１０度から＋１０度の間にあれば、実用的には十分な表示特性を得ることができる。
【０１４１】
また、液晶層４０および位相板４１のねじれ角は、いずれも７０度から１１０度の範囲にあるのが望ましい。これらの場合、２枚の偏光板７，８の偏光軸の方向を上述した方向から若干調整すれば表示特性はさらに良好になる。
【０１４２】
さらに、実際の液晶表示装置では、前述したように、液晶のガンマ特性に伴う信号処理や透明基板上のスイッチング素子の特性が不十分であるため、表示電圧無印加状態で液晶層に若干ながら電圧（オフ電圧）が印加される。このようなオフ電圧により、表示電圧無印加状態でも液晶分子がわずかに立ち上がった状態となりやすい。
【０１４３】
このようなことに鑑みて、本実施の形態の液晶表示装置では、位相板４１のリターデーションは、液晶層４０のリターデーションに等しいか、それよりわずかに小さく設定するのが望ましい。実験によれば、位相板４１のリターデーションを液晶層４０のリターデーションの９５％から１００％の間に設定すれば、ほとんどの信号処理回路やスイッチング素子に対して良好な特性を与えることが分かっている。
【０１４４】
なお、位相板４１のリターデーションを小さくする代わりに、位相板４１が有する負の一軸性光学媒体の主軸を傾ければ、さらに良好な視野角特性を得ることができる。また、第２の実施の形態と同様に、位相板４１の等価的なリターデーションが、液晶層４０のリターデーションの９５％から１００％の間にあれば、ほとんどの信号処理回路やスイッチング素子に対して良好な特性を得ることができる。
【０１４５】
位相板４１が有する負の一軸性光学媒体の主軸のチルト角が位相板厚み方向に分布を持つ場合、負の一軸性光学媒体のチルト角が液晶分子のチルト角に比べて大きすぎる部分があると、その部分の視野角特性が大きくなりすぎるので好ましくない。この場合は、液晶層４０が有する液晶分子のプレチルト角の３倍を越えないように、負の一軸性光学媒体のチルト角の最大値を設定するのが望ましく、液晶分子のプレチルト角の２倍を越えないように、負の一軸性光学媒体のチルト角を設定するのがさらに望ましい。
【０１４６】
また、液晶層４０の表示電圧無印加状態における液晶分子の配列状態が、シミュレーション等によって分かっている場合には、第３の実施の形態で説明したのと同様に、位相板４１が有する負の一軸性光学媒体をこれに対応して配列させれば、さらに良好な視野角特性を得ることができる。
第７の実施の形態
本実施の形態の液晶表示装置は、ＴＮ型の液晶層４０を有するものの、その構成は、基本的には図１３に示すもの（第４の実施の形態）と同様であるので、図１３を参照して説明する。なお、本実施の形態においては、第４の実施の形態と同一ないしは同様の部分には同一の符号を付している。
【０１４７】
本実施の形態の液晶表示装置は、２枚に分割された第１，第２の位相板４１Ａ，４１Ｂを備え、これら位相板４１Ａ，４１Ｂを液晶層４０から見て表裏に配置した構成をとっている。本実施の形態においても、第４の実施の形態で説明したのと同様の原理で、視野角特性が向上するうえ、視野角対称性も向上する。
【０１４８】
なお、液晶層４０と第１，第２の位相板４１Ａ，４１Ｂの近接面における液晶分子や光学媒体の主軸が相互になす角度は、すべての近接面で１０度以内、より望ましくは５度以内にするのがよい。すなわち、液晶層４０の第１の位相板側界面４０ｂにおける液晶分子（図示省略）の配列方向と、第１の位相板６Ａの液晶層側界面４１ｂにおける負の一軸性光学媒体（図示省略）の主軸方向とがなす角度は１０度、より望ましくは５度以内にするのが望ましい。同様に、液晶層４０の第２の位相板側界面４０ｃにおける液晶分子（図示省略）の配列方向と、第２の位相板４１Ｂの液晶層側界面４１ｃにおける負の一軸性光学媒体（図示省略）の主軸方向とがなす角度は１０度、より望ましくは５度以内にするのが望ましい。
【０１４９】
第１、第２の位相板４１Ａ，４１Ｂの負の一軸性光学媒体の主軸がチルト角をもつ場合には、第１の位相板４１Ａと液晶層４０との間、および第２の位相板４１Ｂと液晶層４０との間で互いに近接し合う界面（界面４１ｂと界面４０ｂ、ないしは界面４１ｃと界面４０ｃ）においてチルト角が連続に近くなるように、位相板４１Ａ，４１Ｂの界面４１ｂ，４１ｃにおける負の一軸性光学媒体の主軸方向およびプレチルト角を設定するのが望ましい。
【０１５０】
また、偏光板７，８の配置角については、次のようにすればよい。すなわち、第２の位相板４１Ｂの偏光板８側界面４１ｄにおける負の一軸性光学媒体の主軸方向と偏光板８の偏光軸方向を平行または直交配置し、第１の位相板４１Ａの位相板７側界面４１ａにおける負の一軸性光学媒体の主軸方向と偏光板７の偏光軸方向とを、前者が平行配置の場合には直交配置し、前者が直交配置の場合には平行配置することを基本とする。そして、この基準位置に、他のパラメータに応じて、１０度程度までの範囲で偏光板７，８の配置角度を調整すればよい。
【０１５１】
第１，第２の位相板４１Ａ，４１Ｂのリターデーションとねじれ角については次のようにすればよい。すなわち、第１，第２の位相板４１Ａ，４１Ｂのリターデーションの合計、およびねじれ角の合計が、第６の実施の形態で１枚の位相板４１に対して説明した条件を満たせばよい。なお、第１，第２の位相板４１Ａ，４１Ｂはリターデーション等の光学特性が必ずしも等価になるように分割しなくても、ここで説明した効果は発揮される。
第８の実施の形態
本実施の形態の液晶表示装置は、ＴＮ型の液晶層４０を有しているものの、その構成は基本的には、図１５に示す第５の実施の形態の液晶表示装置と同じものであるので、図１５を参照して説明する。なお、本実施の形態においては、第５の実施の形態と同一ないしは同様の部分には同一の符号を付している。
【０１５２】
本実施の形態の液晶表示装置の特徴は、ＴＮ型の液晶層４０とは逆方向にねじれた負の屈折率異方性をもつ一軸性光学媒体（負の一軸性光学媒体）を備えた位相板（負の位相板）４１に加えて、液晶層と同一方向にねじれた正の屈折率異方性をもつ一軸性光学媒体（正の一軸性光学媒体）を備えた位相板（正の位相板）４３を有している。この位相板４３の構成は第５の実施の形態で説明した正の位相板１８と同様の構成を備えている。
【０１５３】
本実施の形態に示した構成においても、第５の実施の形態で説明したのと同様の原理で、正の位相板４３が液晶表示特性の方位角依存を緩和あるいは平均化し、負の位相板４１がこの視野角依存を補償するので、第６の実施の形態に比べて視野角特性がさらに向上する。
【０１５４】
第５の実施の形態では、液晶層１のねじれ角と正の位相板１８のねじれ角との合計を３６０度程度に設定した。本実施の形態でも、液晶層４０のねじれ角と正の位相板４３のねじれ角との合計を３６０度程度に設定すれば視野角依存性を各方位に分散させることができるが、ＴＮ型液晶層４０はねじれ角が約９０度とＳＴＮ型の液晶層１に比べて狭いので、この合計のねじれ角が１８０度以上であれば実用上十分な視野角拡大効果を得ることができる。
【０１５５】
本実施の形態の構成では、液晶層４０、位相板４３，４１のねじれ角の関係が次の（９）式の範囲にあることが望ましい。
【０１５６】
−１０°≦ΩLC＋Ωp＋Ωn≦１０°…（９）
ΩLC：液晶層４０を構成する液晶分子のねじれ角
Ωp：正の位相板４３を構成する正の一軸性光学媒体のねじれ角
Ωn：負の位相板４１を構成する負の一軸性光学媒体のねじれ角
なお、ねじれ方向は、左ねじれを正、右ねじれを負としており、位相板４３，４１のリターデーションは、位相板４３，４１が有する一軸性光学媒体が傾斜配列をもつ場合には、等価的なリターデーションとしている。
【０１５７】
次に、液晶層４０と位相板４１，４３との間のリターデーションの望ましい関係について説明する。本実施の形態では、正の位相板４３は液晶層４０の延長とみなされる構造をもつのが望ましいので、そのリターデーションＲpは次に示す
（１０）式で求められるＲpoの値を基本として考えればよい。
【０１５８】
Ｒpo＝ＲLC×Ωp／ΩLC…（１０）
ＲLC：液晶層４０のリターデーション
Ωp：正の位相板４３を構成する正の一軸性光学媒体のねじれ角
ΩLC：液晶層４０を構成する液晶分子のねじれ角
第６の実施の形態で説明したように、アクティブマトリクス型の液晶表示装置においては、表示電圧無印加状態（オフ状態）でも液晶分子がわずかに立ち上がった状態となることが多い。このため、正の位相板４３のリターデーションを上記した（１０）式で定まる値より２〜３％程度小さく設定したり、正の位相板４３を構成する正の一軸性光学媒体をチルト配列させたりすれば、より良好な表示特性が得られる。
【０１５９】
なお、正の位相板４３を構成する正の一軸性光学媒体のチルト配列の方向は液晶分子のチルト方向と一致させるのがよい。
【０１６０】
本実施の形態の構成においては、負の位相板４１のリターデーションのうち一部分が正の位相板４３の特性を補償し、負の位相板４１の残りの部分のリターデーションが液晶層４０の特性を補償するものと考えられる。従って、負の位相板４１のリターデーションのうち液晶層４０の特性を補償する部分は、Ｒneff−Ｒpeffとなる。第６の実施の形態で説明したのと同様の理由で、Ｒneff−ＲpeffとＲLCの間に、次に示す（１１）式の関係が成り立つのが望ましい。
【０１６１】
０.９５≦（Ｒn−Ｒpeff）／ＲLC≦１…（１１）
また、本実施の形態においても、液晶層４０と各位相板４３，４１との間において、その互いに近接し合う面における液晶分子や正負の一軸性光学媒体の主軸が相互になす角度は、すべての近接面で１０度以内にするのが望ましく、５度以下にするのがさらに望ましい。さらには、位相板４３，４１における正負の一軸性光学媒体の主軸がチルト角をもつ場合には、液晶層４０，位相板４３，４１の液晶分子および正負の一軸性光学媒体の主軸のチルト角および配向方向を次のように設定するのが望ましい。すなわち、液晶層４０と位相板４３との間、および位相板４３と位相板４１との間において、互いの近接面でチルト角が連続に近くなるように、上記したチルト角および配向方向を設定するのが望ましい。
【０１６２】
また、偏光板７，８の配置角については、次のように設定する。すなわち、偏光板８の偏光軸とそれに隣接する液晶層１では、その近接面側において、偏光板８の偏光軸と液晶層１の液晶分子の主軸方向とが平行または直交で、偏光板７の主軸方向は偏光板８の主軸方向と直交する配置から（９）式で示されたねじれ角の総和の分だけずらした配置を基本とする。そして、このような基本配置を基にして、他のパラメータに応じて、１０度程度までの範囲で偏光板７，８の配置角度を調整すればよい。液晶層４０のねじれ角やリターデーションについては第６の実施の形態と同様の範囲に設定するのが望ましい。
第８の実施の形態の変形例
第８の実施の形態の液晶表示装置において、正負の位相板４３，４１のいずれか一方または双方を２枚の位相板に分割し、液晶層４０の上下に１枚づつ配置したところ、さらに良好な表示特性を得ることができた。
【０１６３】
これは、第５の実施の形態の変形例３で説明したのと同様の理由によると考えられ、どちらかといえば負の位相板４１を分割する方が、より大きな視野角特性向上効果が得られる。
【０１６４】
なお、液晶層４０と各位相板４３，４１の近接面における液晶分子や正負の一軸性光学媒体の主軸とが相互になす角度や偏光板７，８の配置角については、第８の実施の形態に述べた範囲に設定すればよい。
【０１６５】
次に、上記した各実施の形態で用いられる負の屈折率異方性を有する位相板の製造方法について説明する。
【０１６６】
負の屈折異方性をもつ光学媒体をねじれ配列させた位相板は、高分子ネットワーク内にネマティック液晶を配列させた光学層を形成した後に、ネマティック液晶をディスコティック液晶に置換することにより作製することができる。これは、方向制御やねじれ構造の付与が困難なディスコティック液晶を、比較的容易に配列制御できるネマティック液晶と置換することにより、所望の配列分布させるものである。より具体的で好ましい作製方法としては、図１８に工程図を示す方法がある。
【０１６７】
まず、１８（ａ）に示すように、例えばセルロース・トリアセテートなどの光学的に等方的なフィルム基材１０１の表面を、ラビングロール１０２を用いたラビング処理などにより配向処理する。このとき、フィルム基材１０１表面にポリイミドなどの配向膜を形成しておいてもよい。
【０１６８】
次いで、図１８（ｂ）に示すように、ネマティック液晶と光重合型の高分子との混合物１０３を、ロールコート法によりフィルム機材１０１上に塗布する。図中、符号１０４は混合物１０３の塗布を行うロールコータのロールである。混合物１０３は必要により溶媒に溶かしても構わない。また、混合物１０３の塗布はスピンコートや各種の印刷法を用いてもよい。
【０１６９】
このようにして、光重合型の高分子とともにフィルム基材１０１に塗布するネマティック液晶には若干のカイラル物質（例えばメルク社製のＳ−８１１）を添加しておき、ねじれ構造をもたせておく。一方、光重合型の高分子としては紫外線硬化型のアクリル樹脂などを用いる。混合物１０３を溶媒に溶かしておく場合には、次に説明する工程の前に乾燥により溶媒をとばしておく。
【０１７０】
次に図１８（ｃ）に示すように、混合物１０３に紫外線光１０５を照射することにより、光重合型の高分子を重合させて混合物１０３内に高分子ネットワークを形成する。このように形成された高分子ネットワークにより、ねじれ構造を有するネマティック液晶分子の配列が安定化する。なお、照射する光は光重合型の高分子がネットワークを形成する波長であれば可視光でもかまわない。
【０１７１】
その後、図１８（ｄ）に示すように、メタノール、エタノール等のネマティック液晶を溶解させる溶媒１０６を用意し、この溶媒１０６に、高分子ネットワークが形成されたフィルム基材１０１を浸析し、これにより、フィルム基材１０１上の混合物１０３からネマティック液晶を選択的に除去する。浸析する代わりに、フィルム基材１０１上に溶媒１０６をシャワー状に注いでも、ネマティック液晶を選択的に除去することができる。
【０１７２】
次いで、図１８（ｅ）に示すように、溶媒１０６中から引き上げたフィルム基材１０１上に、シリンジ１０９によりディスコティック液晶１０８を滴下する。ディスコティック液晶１０８としては、例えば、トリフェニルレン系の化合物、ベンゼン環の側鎖として長鎖型あるいは板状の官能基が配置されたベンゼン誘導体を用いる。
【０１７３】
ディスコティック液晶１０８は、ネマティック液晶の除去により空孔が形成された高分子ネットワーク１０７に浸入して分散する。高分子ネットワーク１０７に形成されているネマティック液晶跡の空孔にはネマティック液晶のねじれ構造に対応した異方性を備えているので、この空孔に浸透するディスコティック液晶１０８にもねじれ構造および異方性が与えられる。このねじれ構造の形状は、上述したネマティック液晶へのカイラル物質の添加量を調整することにより制御することができる。なお、このような工程においては、シリンジ１０９やフィルム１０１基材の加熱によりディスコティック液晶１０８の流動性を高めれば、プロセス時間の短縮を図ったり、室温では液晶相をとらない物質を用いたりすることができる。
【０１７４】
また、シリンジ１０９による滴下操作の代わりに、ディスコティック液晶１０８の中にフィルム基材１０１を浸析したり、フィルム基材１０１上にディスコティック液晶１０８をロールコート法や印刷法により塗布してもよい。さらには、ディスコティック液晶１０８単体を用いる代わりに、ディスコティック液晶１０８を適当な溶媒に溶かしたものを用意し、この溶媒に溶かし込んだディスコティック液晶１０８をフィルム基材１０１上に滴下ないし塗布してもよい。この場合には、フィルム基材１０１にディスコティック液晶１０８を浸透させたのち、溶媒を除去する必要が生じる。
【０１７５】
次いで、図１８（ｆ）に示すように、余分なディスコティック液晶１０８を除去する。これにより、ねじれ構造をもったディスコティック液晶１０８が高分子ネットワーク１０７内に分散してなる光学層１１０が形成され、この光学層１１０を有するフィルム基材１０１が、上述した負の屈折率異方性を有する位相板となる。なお、高分子ネットワーク１０７の添加量は、重量比にして２〜３％程度にしておくのが望ましい。そうすれば、ディスコティック液晶１０８に対して光学的影響が小さくなる。
【０１７６】
このように形成した光学層１１０中のディスコティック液晶１０８の流動性が低く、位相板の物理的安定性が良好な場合は、このまま位相板として用いても構わない。しかしながら、そうではなく、ディスコティック液晶１０８の流動性が高い場合には、光学層１１０の表面に、例えばセルロース・トリアセテート膜など、光学層１１０に対して光学的に等方的な保護フィルム膜を形成して、光学層１１０を保護すれば、位相板の光学的安定度や、機械的安定度ないし強度が増加する。また、偏光板や他の位相板を光学層１１０上に積層する場合には、これらの薄板を上述した保護フィルム膜の代用とすることもできる。また、ディスコティック液晶１０８の流動性が高い場合には、光学層１１０およびフィルム基材１０１を所定の大きさに裁断した後、その裁断面を封止するのが望ましい。
【０１７７】
なお、図１８を参照した説明した製造方法では、フィルム基材１０１上に光学層１１０を形成して位相板としているが、フィルム基材１０１に代えて位相板、偏光板、または液晶パネルの表面に同様の光学層を形成しても構わない。
【０１７８】
さらに、この製造方法は、図１９の工程概念図を示すように、ロール状のフィルム基材１０１'に連続的に位相板を形成するのに特に適している。なお、図１９では、図１８と同一ないし同様の部分には同一の符号を付しおり、それらについての詳細な説明は省略する。
【０１７９】
フィルム基材１０１'は送り出し側のロール１２１から連続的に供給され、図１８で説明したのと同様のプロセスを経て、その表面に光学層１１０（図１９では図示省略）が形成されたのち受け取り側のロール１２２に巻きとられる。
【０１８０】
光学層１１０が形成されたフィルム基材１０１'は、この後すぐに、裁断等の加工を行っても構わないし、ロールの形状で位相板、偏光板等の他のフィルムと貼合せしてもよい。このように連続生産を行えば生産量が大幅に拡大し、負の屈折率異方性を有する位相板を安価に大量に供給できる。
【０１８１】
このようにして製造した負の屈折率異方性を有する位相板は、連続的なねじれ構造を持つため光学的な特性が良好であり、これを第１から第８の実施の形態の液晶表示装置に用いたところ非常に良好な表示特性を得ることができる。また、ロール状のフィルムを用いた連続生産を行えば、さらに液晶表示装置の低価格化に寄与することができる。
【０１８２】
上述した負の屈折率異方性を有する位相板の製造方法において、ディスコティック液晶１０８として、末端に反応基が形成されたディスコティック液晶１０８'を用いてもよい。すなわち、上述したディスコティック液晶１０８'を、図１８（ｆ）と同様、ネマティック液晶の除去により空孔が形成された高分子ネットワーク１０７に浸透させる。さらには、ディスコティック液晶１０８'の相互間、および高分子ネットワーク１０７とディスコティック液晶１０８'との間で架橋反応させる。架橋反応は、加熱や光照射により生じさせることができる。
【０１８３】
このようにして、ディスコティック液晶１０８'の相互間、および高分子ネットワーク１０７とディスコティック液晶１０８'との間で架橋させることで、安定性の良好な光学層１１０'、ひいては位相板を得ることができる。この場合は、位相板側面を封止する必要はない。また、特に位相板の上面（または下面）をキズ等から保護する必要がない場合には、ディスコティック液晶１０８'を有する光学層１１０'を保護フィルムを覆わなくてもよい。
【０１８４】
また、架橋により安定性を固めた位相板の製造方法において、さらに次のようにしてもよい。すなわち、図１８（ｆ）の構成を作成したのち、高分子材料やディスコティック液晶１０８'の末端基の選定や架橋反応条件の設定により、ディスコティック液晶１０８'相互のみに架橋反応を起こさせる。
【０１８５】
その後、図２０（ａ）に示すように、トルエン、キシレンといった、光学層１１０'の中にある高分子ネットワーク１０７のみを選択的に溶解させる溶媒１１１により光学層１１０'から高分子ネットワーク１０７だけを選択的に除去する。これにより、ディスコティック液晶１０８'のみからなる光学層１１２をフィルム基材１０１上に形成する。
【０１８６】
次いで、図２０（ｂ）に示すように、末端に反応基が形成されたディスコティック液晶１１４を用意し、このディスコティック液晶１１４をシリンジ１１３を用いた滴下操作等により、光学層１１２上に供給し、このディスコティック液晶１１４を光学層１１２内に形成された高分子ネットワーク１０７跡の空孔に浸透させる。さらに、このディスコティック液晶１１４を架橋させて、図２０（ｃ）に示す光学層１１５を形成する。
【０１８７】
このようにして得られる位相板は、図１８（ｆ）に示す光学層１１０の高分子ネットワーク１０７がディスコティック液晶１１４に置換され、光学層１１５全体がディスコティック液晶から形成されたものになる。したがって、このようにして製造された位相板は、次のような利点がある。すなわち、図１８を参照して説明した製造方法で作成された位相板の機械的安定性を高めるためには、光学層１１０内の高分子ネットワーク１０７の量を増加させる必要がある。しかしながら、高分子ネットワーク１０７を増加させると、それに応じて、ディスコティック液晶１０８に対する光学的な悪影響（例えば、屈折率の違いによる光の散乱等で変化すること）が増大する。つまり、図１８の製法で作成された位相板では、光学特性と機械的安定性とは相反する関係にある。
【０１８８】
これに対して、光学的ネットワーク１０７をディスコティック液晶１１４に置換させる製法では、その分、工程が増加するものの、高分子ネットワーク１１４の光学的悪影響を排除することができ、光学特性と機械的安定性とを両立させることができる。
【０１８９】
上述した位相板の製造方法それぞれにおいて、ネマティック液晶と光重合型の高分子との混合物１０３に光照射して高分子ネットワークを形成する際に、フィルム法線方向に電界（または磁界）を印加しながら光照射を行ってもよい。そうすれば、ネマティック液晶の分子がフィルム基材１０１の法線方向に立ち上がるので、最終的に得られるディスコティック液晶１０８の主軸方向を、フィルム基基材１０１の法線方向から傾けたり、フィルム基材１０１の深さ方向にチルト角の分布を持たせたりすることができる。
【０１９０】
これにより、第２の実施の形態などで説明した位相板、すなわち、負の一軸性光学媒体１５の主軸方向を、液晶層１の分子配列に整合させた位相板を得ることができる。
【０１９１】
なお、上述した負の屈折率異方性を有する位相板の製造方法で作成された位相板では、フィルム基材１０１に接している光学層１１０の界面と、露出している光学層１１０の界面とでは、チルト角が互いに異なることがあり、表示装置の光学特性から対称的なチルト構造が要求される場合には、このような構成は都合が悪い。
【０１９２】
そこで、対称的なチルト構造が要求される場合には、上述した負の屈折率異方性を有する位相板の製造方法で作成された位相板の複数枚を、互いに界面で負の一軸性光学媒体の主軸がほぼ連続するように積層して１枚の位相板とすることにより、対称的なチルト構造を得ることができる。
【０１９３】
なお、この場合には、両端以外の位相板では、フィルム基材１０１を取り除くのが、全体としての位相板の光学特性を高めたり、位相板の厚みを増加させないうえで望ましい。また、積層枚数が多い場合には界面の影響で表示特性が劣化するので、積層枚数は３枚以下にすることが望ましい。
【０１９４】
さら上述した位相板の製造方法では、ディスコティック液晶１０８をねじれ配列させることにより、主軸がねじれ配列した負の位相板を作製するものとした。しかしながら、これらの製造法において、最初に作成するネマティック液晶の構造を、ねじれ配列とは異なる別の配列をとるように制御しておけば、他の配列の負の位相板を作製することも可能である。例えば、カイラル剤を添加しなければ、界面での配向条件や、光硬化時の電界や磁界の印加条件により、ネマティック液晶を垂直配向，水平配向，傾斜配向，ないしはハイブリッド配向させることができる。これにより、光学軸が垂直配向，水平配向，傾斜配向，ないしはハイブリッド配向した負の位相板を作製することができる。
【０１９５】
【発明の効果】
以上のように、本発明によれば、表示電圧無印加状態（オフ状態）にある液晶層の光学伝搬特性の視野角依存を補償し、視野角の広い液晶表示装置を得ることができるうえ、特に、ＴＮ型液晶表示装置に適用した場合には、ＮＢ表示での視野角特性が十分に良好な表示を行うことができるため、駆動電圧の低減、特に３.３ボルト駆動が可能になった。
【０１９６】
また、本発明によれば、負の光学媒質の光学軸がねじれ配列した位相板を容易に形成することができた。
【０１９７】
以上が本発明の全体的な効果である。次に、各請求項の効果を詳細に述べる。
請求項１の効果
電圧無印加状態にある液晶層の光学伝搬特性の視野角依存が補償され、これによって視野角特性が改善され、かつ、表示電圧無印加状態にある液晶層の光学伝搬特性の視野角依存が方位角方向に分散し、視野角依存が補償された。そのため、その分、さらに視野角特性が向上した。また、液晶層が単純マトリクス構造でよくなり、その分、視野角特性のよい液晶表示装置を低価格で提供することができるようになって、コストダウンが図れた。また、表示電圧無印加状態にある液晶層における光学伝搬特性の視野角依存がすべての方位角でほぼ均等に分散されることになり、方位角による視野角特性の差が小さくなり、その分、さらに視野角特性が向上した。
請求項２の効果
液晶層と第２の位相板との視野角補償効果が高まり、その分、さらに視野角特性が向上した。
請求項３の効果
第１の位相板と第２の位相板との視野角補償効果が高まり、その分、さらに視野角特性が向上した。
請求項４の効果
光学補償の裕度が高まり、光学伝搬特性の対称性が高まるうえに、液晶層と第２の位相板との視野角補償効果もさらに高まった。これにより、さらに視野角特性が向上した。
請求項５の効果
液晶層と第１，第２の位相板の光学伝搬特性の整合性が高まり、その分、さらに視野角特性が向上した。
請求項６の効果
アクティブマトリクスとの組み合わせにより、高コントラストになるとともに視野角特性も向上した。また、表示電圧無印加状態にある液晶層の視野角依存が広く分し、方位角による視野角特性の差が小さくなり、視野角依存性が小さくなった。
請求項７の効果
液晶層と第１、第２の位相板との光学伝搬特性の整合性が高まり、その分、視野角特性がさらに向上した。
【図面の簡単な説明】
【図１】 本発明の第１〜第３の実施の形態の液晶表示装置の構成を示す断面図である。
【図２】 本発明の液晶表示装置における液晶層の摸式的な断面図である。
【図３】 本発明の液晶表示装置における位相板の摸式的な断面図である。
【図４】 本発明の液晶表示装置における液晶分子と、負の屈折率異方性を有する一軸性光学媒体との配列関係を摸式的に示す断面図である。
【図５】 負の屈折率異方性を有する一軸性光学媒体と正の屈折率異方性を有する一軸性光学媒体との視野角補償効果の比較に供する断面図である。
【図６】 位相板と液晶層のリターデーション比に対するコントラストの関係を示す特性図である。
【図７】 第１の実施の形態の液晶表示装置の視野角特性図である。
【図８】 正の屈折率異方性を有する一軸性光学媒体の位相板を用いた従来の液晶表示装置の視野角特性図である。
【図９】 本発明の第２の実施の形態の液晶表示装置を構成する位相板の模式的な断面図である。
【図１０】 液晶層の分子配列状態のシミュレーション結果を示す図である。
【図１１】 位相板の光学軸の配列状態の設計条件を示す図である。
【図１２】 本発明の第３の実施の形態の液晶表示装置における液晶分子と、負の屈折率異方性を有する一軸性光学媒体との配列関係を摸式的に示す断面図である。
【図１３】 本発明の第４の実施の形態の液晶表示装置の構成を示す断面図である。
【図１４】 第４の実施の形態の液晶表示装置の視野角特性図である。
【図１５】 本発明の第５の実施の形態の液晶表示装置の構成を示す断面図である。
【図１６】 第５の実施の形態における液晶分子、負の屈折率異方性を有する一軸性光学媒体、および正の屈折率異方性を有する一軸性光学媒体の配列関係を摸式的に示す断面図である。
【図１７】 第５の実施の形態の液晶表示装置の視野角特性図である。
【図１８】 本発明の位相板の製造方法の一実施の形態を示す工程図である。
【図１９】 本発明の位相板の製造方法の変形例を示す工程図である。
【図２０】 本発明の位相板の製造方法のさらに他の変形例を示す工程図である。
【図２１】 従来例の液晶表示装置の構成を示す断面図である。
【符号の説明】
１，４０ 液晶層
６,６Ａ,６Ｂ,１４,１６,１８,４１ 負の一軸性光学媒体を有する位相板
１１ 液晶分子
１２，１５，１７ 負の一軸性光学媒体
１８ 正の一軸性光学媒体を有する位相板
１９ 正の一軸性光学媒体[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a liquid crystal display device that displays video and character information, a phase difference plate used therefor, and a method of manufacturing a phase plate.
[0002]
[Prior art]
  Liquid crystal display devices are widely used as flat displays such as flat-screen TVs, car navigation systems, personal computers, and word processors because of their thin and light features.
[0003]
  A number of display modes have been proposed for liquid crystal displays, but currently widely used are twisted nematic type (hereinafter abbreviated as TN type) and super twisted nematic type (hereinafter abbreviated as STN type). There are two types. The former is mainly used for an active matrix type liquid crystal display device in which a switching element such as a thin film transistor (TFT) is formed for each pixel, and the latter is used for a simple matrix type liquid crystal display device having no switching element in a pixel.
[0004]
  The STN type liquid crystal display device uses a liquid crystal layer and two polarizing plates arranged at a twist angle of two hundred and several tens of degrees, and the liquid crystal molecules rise when a voltage is applied in the normal direction of the substrate, and the birefringence of the liquid crystal layer changes. Polarization display is performed based on the phenomenon (see Japanese Patent Application Laid-Open No. 60-107020). Since the display is colored in the STN type liquid crystal display device, a technique for obtaining a monochrome display by adding a phase plate to remove the coloration (coloring) of the display is widely used. Examples of the phase plate for removing color include a second liquid crystal panel having a structure twisted in the direction opposite to the liquid crystal layer and a phase plate using a polymer liquid crystal (see Japanese Patent Laid-Open No. 63-149624).
[0005]
  FIG. 21 shows a configuration of a conventional STN type liquid crystal display device (hereinafter referred to as a first conventional example) in which display coloring is prevented. In the figure, 304 is a liquid crystal layer, and 303 and 305 are glass substrates. Although not shown, an electrode for applying a voltage to the liquid crystal layer 304 is formed on the liquid crystal layer side of the two glass substrates 303 and 305, and the overlapping portion of these electrodes forms a pixel. Reference numerals 301 and 306 are polarizing plates for performing polarization display, 302 is a phase plate, and 307 is a back light source.
[0006]
  The phase plate 302 plays the role of compensating for the wavelength dependence of the polarization state of the light that has passed through the liquid crystal layer 304, eliminating the wavelength dependence of the polarization that passes through the upper polarizing plate 301, and obtaining a monochrome display. The retardation of the phase plate 302 and the liquid crystal layer 304 are equal to each other, the twist direction is opposite and the twist angle is substantially equal, and the angle formed by the optical axis direction of the phase plate 302 and the rubbing direction of the liquid crystal layer 304 at the interface between them. Is 90 degrees. The phase plate 302 can perform the same operation even if it is formed between the glass substrate 305 and the lower polarizing plate 306, or can be formed on both sides of the liquid crystal layer 304.
[0007]
  In the first conventional example, the display in the front direction is changed to black and white, but the birefringence amount of the liquid crystal layer 304 and the phase plate 302 varies depending on the observation direction. Since the amount of change in birefringence is not mutually compensated, the display characteristics of the liquid crystal panel have a viewing angle dependency such as display reversal and hue change accompanying the viewing angle change. In other words, this conventional example has the advantage that the clear viewing direction is front and the viewing angle characteristics are concentric, but does not specifically have a configuration for widening the viewing angle.
[0008]
  On the other hand, as disclosed in Japanese Patent Laid-Open No. 4-43318, an STN type liquid crystal display device (hereinafter referred to as a second conventional example) having a viewing angle characteristic improved with a configuration using a twisted phase plate has been conventionally used. is there. In the second conventional example, a phase plate twisted in the same direction as the liquid crystal layer is laminated to a total twist angle of 360 degrees, and a phase plate twisted 360 degrees in the opposite direction is further laminated. The viewing angle characteristics are improved.
[0009]
  In the second conventional example, the birefringence amount of the liquid crystal molecules has a viewing angle characteristic with the molecular long axis direction as the symmetric axis, apparently increases the twist angle of the liquid crystal layer, and also increases the twist angle of the phase plate. The viewing angle dependence is improved by making the alignment direction and the principal axis direction of the liquid crystal molecules and the phase plate face in all directions.
[0010]
  However, in the second conventional example, the viewing angle characteristics of the liquid crystal layer and the phase plate are dispersed in each direction to alleviate the viewing angle dependence of the display. However, the reverse twisted phase plate provides optical compensation in the front direction. Since the main focus is on obtaining a monochrome display and not having a viewing angle compensation effect, sufficient viewing angle characteristics could not be obtained.
[0011]
  On the other hand, a TN type liquid crystal display device has a configuration in which liquid crystal molecules are arranged at a twist angle of 90 degrees (see Japanese Patent Application Laid-Open No. 55-52027). In such a configuration, when light having a polarization plane parallel or perpendicular to the liquid crystal molecules is incident on the substrate perpendicularly, the light is emitted with the polarization plane rotated by 90 degrees. When a voltage in the substrate normal direction is applied to the liquid crystal layer, the liquid crystal molecules rise and their twisted structure is eliminated, so that the liquid crystal loses its action on the polarized light, and the incident polarized light is emitted as it is.
[0012]
  Therefore, in the TN liquid crystal display device, black and white display is obtained by rotating the polarization plane of the emitted light by 90 degrees by turning on and off the voltage and using two polarizing plates. The two polarizing plates are arranged so that the deflection axes are parallel (or perpendicular) to the principal axis direction of the adjacent liquid crystal molecules. When the polarizing axes of the two polarizing plates are orthogonal, no display voltage is applied. White, normally white (NW) display in which black is applied when the display voltage is applied, and when the polarization axes of the two polarizing plates are parallel, black when the display voltage is not applied and white when the display voltage is applied Mary black (NB) display.
[0013]
  Also in the case of a TN liquid crystal display device, since the polarization propagation characteristics of the liquid crystal layer differ depending on the viewing direction, the display characteristics of the liquid crystal panel have, for example, viewing angle dependency such as display inversion and hue change accompanying a viewing angle change. Arise. Since a TN type liquid crystal display device using an active matrix often displays halftone images such as video, improving the viewing angle characteristics in the vicinity of the black display, which has a particularly large effect, is a performance issue. Yes.
[0014]
  Therefore, conventionally, as shown in JP-A-6-214116, using a phase plate in which a birefringent medium having a negative refractive index anisotropy is disposed so that its optical axis is inclined from the sheet normal direction, There is an NW display TN liquid crystal display device with improved viewing angle characteristics (hereinafter referred to as a third conventional example). Further, as disclosed in JP-A-7-13021, a birefringent medium having a negative refractive index anisotropy is used with a phase plate in which the optical axis is inclined from the normal direction of the sheet. In some cases, the viewing angle characteristics of a TN type liquid crystal display device for NW display are improved by stacking (hereinafter referred to as a fourth conventional example).
[0015]
  In the third and fourth conventional examples, the viewing angle dependence at the time of applying the display voltage is compensated by the phase plate having the above-described configuration, thereby obtaining good black display characteristics and improving the viewing angle characteristics. In addition, as a method of producing a phase plate with an inclined optical axis, a method of obliquely cutting a film from a bulk polymer having negative birefringence, or a mixture of a polymer matrix and a low-molecular liquid crystal and arranging a low-molecular liquid crystal diagonally The low molecular liquid crystal preferably exhibits negative birefringence.
[0016]
  However, the third and fourth conventional examples compensate for the characteristics of the liquid crystal layer when a display voltage is applied in which the liquid crystal molecular arrangement is greatly deformed. It is necessary to sufficiently raise the liquid crystal molecules in the center of the liquid crystal layer by applying the voltage of For this reason, when a sufficient voltage cannot be applied to the liquid crystal layer, good viewing angle characteristics cannot be obtained, or the voltage level range where good compensation is performed is narrow, making it difficult to set driving conditions. That is, at present, electronic devices are actively moving to reduce the logic voltage from 6 volts to 3.3 volts in order to reduce power consumption and improve signal processing speed. Under such circumstances, even if the configurations of the third and fourth conventional examples are employed in the TN type liquid crystal display device for NW display, it is difficult to obtain good display characteristics by driving the signal voltage of 3.3 volts. Met. That is, in the third and fourth conventional examples, the response speed is lowered due to the use of the high-viscosity low-voltage liquid crystal, and the contrast and viewing angle characteristics are lowered due to insufficient applied voltage. .
[0017]
  In the fourth conventional example, the phase plate is formed, for example, by laminating eight films in which linearly polarized light is incident and the photoisomerized material is oriented, shifted by 10 degrees. In this configuration, the number of steps increases, which causes an increase in cost, and there is a disadvantage that sufficient phase plate characteristics cannot be obtained due to optical scattering on the bonding surface, and the contrast of the liquid crystal display device is lowered. .
[0018]
  Furthermore, in the fourth conventional example, the main purpose is to improve the viewing angle characteristics of the TN type liquid crystal display device for NW display. For this purpose, optically negative uniaxiality or biaxiality close to uniaxiality is used. A phase plate obtained by laminating thin films having a twisted structure is combined with a TN liquid crystal. As can be seen from the above, the fourth conventional example is intended for a liquid crystal display device that performs NW display, but is not intended for a TN type or STN type liquid crystal display device that performs NB display. It is unclear whether it can be implemented with a liquid crystal display device.
[0019]
  On the other hand, when performing NB display with a TN liquid crystal display device, black display is performed when no display voltage is applied. According to a detailed calculation, the condition in which the incident linearly polarized light is emitted by rotating the plane of polarization exactly 90 degrees is established only for a specific wavelength, so that the black display is slightly colored even in the front direction.
[0020]
  In order to eliminate such coloring, a panel having a reverse twisted liquid crystal layer is conventionally laminated as shown in Japanese Patent Application Laid-Open No. 57-96315, thereby eliminating the coloring in the front direction and black display characteristics. (Hereinafter referred to as the fifth conventional example). However, in the fifth conventional example, the viewing angle improvement characteristic is not described, and a sufficient viewing angle cannot be obtained.
[0021]
[Problems to be solved by the invention]
  As described above, in each of the conventional examples described above, the display characteristics of the liquid crystal display device can be improved without changing the basic configuration of the liquid crystal layer, but the display characteristics when the liquid crystal display device is viewed from an oblique direction are not satisfactory. In the TN liquid crystal display device, there is a problem that sufficient viewing angle characteristics cannot be obtained unless the voltage applied to the liquid crystal layer is high.
[0022]
  In addition, a twisted phase plate is formed by laminating linear phase plates, and accordingly, the number of manufacturing steps increases, and the contrast characteristics of the liquid crystal display device deteriorate due to optical scattering on the bonding surface. There was also a problem.
[0023]
[Means for Solving the Problems]
  In order to achieve the above object, the present invention has the following configuration. That is, the present inventionA liquid crystal layer having a positive refractive index anisotropy and a liquid crystal twist angle of 200 degrees or more and 300 degrees or less, at least one film-like first phase plate laminated on the liquid crystal layer, and the liquid crystal And at least one film-like second phase plate laminated in a layer, the first phase plate having negative refractive index anisotropy, and a main axis of the liquid crystal molecules of the liquid crystal layer The second phase plate has a positive refractive index anisotropy and the principal axis of the liquid crystal molecules of the liquid crystal layer. A uniaxial molecule that is twisted in the same direction as the twist direction and has a total liquid crystal twist angle of 330 to 390 degrees.The liquid crystal display device is characterized by having the liquid crystal display device.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
  The invention described in claim 1 of the present inventionA liquid crystal layer having a positive refractive index anisotropy and a liquid crystal twist angle of 200 degrees or more and 300 degrees or less, at least one film-like first phase plate laminated on the liquid crystal layer, and the liquid crystal And at least one film-like second phase plate laminated in a layer, the first phase plate having negative refractive index anisotropy, and a main axis of the liquid crystal molecules of the liquid crystal layer The second phase plate has a positive refractive index anisotropy and the principal axis of the liquid crystal molecules of the liquid crystal layer. A uniaxial molecule that is twisted in the same direction as the twist direction and has a total liquid crystal twist angle of 330 to 390 degrees.Have. This has the following effects. That is, the first phase plate compensates the viewing angle dependence of the optical propagation characteristics of the liquid crystal layer in the display voltage non-applied state or the off-voltage applied state, and the optical propagation characteristics of the liquid crystal layer in the display voltage non-applied state. The viewing angle dependence is dispersed in the azimuth direction, and the viewing angle dependence is made more uniform.In addition, the liquid crystal layer may have a simple matrix structure, and accordingly, a liquid crystal display device having good viewing angle characteristics while being inexpensive can be provided at a low price. In addition, the viewing angle dependence of the optical propagation characteristics in the liquid crystal layer in the off-voltage application state is almost uniformly distributed in all azimuth angles, and the difference in viewing angle characteristics due to azimuth angles is reduced.
[0025]
  According to a second aspect of the present invention, in the liquid crystal display device according to the first aspect, the liquid crystal molecule alignment direction at the second phase plate side interface of the liquid crystal layer and the liquid crystal layer side interface of the second phase plate.Uniaxial molecularThe angle formed with the main axis direction is 10 degrees or less, thereby having the following action. That is, the viewing angle compensation effect between the liquid crystal layer and the second phase plate is enhanced.
[0026]
  According to a third aspect of the present invention, in the liquid crystal display device according to the first or second aspect, at the second phase plate side interface of the first phase plate.Disk-shaped moleculeAt the first phase plate side interface of the main axis direction and the second phase plateUniaxial molecularThe angle formed with the main axis direction is 10 degrees or less, and thus has the following action. That is, the viewing angle compensation effect between the first phase plate and the second phase plate is enhanced.
[0027]
  Of the present inventionThe invention according to claim 4 provides the first to third aspects.In the liquid crystal display device according to any one of the above, the second phase plate is disposed on both surfaces of the liquid crystal layer, thereby having the following action. That is, the tolerance of optical compensation increases and the symmetry of optical propagation characteristics increases. Furthermore, the viewing angle compensation effect between the liquid crystal layer and the second phase plate is further enhanced.
[0029]
  Of the present inventionClaim 5The invention described inClaim 1A liquid crystal twist angle of the liquid crystal layer,Disk-shaped moleculeThe main shaft twist angle,Uniaxial moleculeThe sum of the main shaft twist angle and the sign with one of the twist directions being positive is −10More than +10This has the following effects. That is, the consistency of the optical propagation characteristics of the liquid crystal layer and the first and second phase plates is enhanced.
[0031]
  Claims of the invention6The invention described inA liquid crystal layer having a positive refractive index anisotropy and a liquid crystal twist angle of 70 degrees or more and 110 degrees or less, at least one film-like first phase plate laminated on the liquid crystal layer, and the liquid crystal And at least one film-like second phase plate laminated in a layer, the first phase plate having negative refractive index anisotropy, and a main axis of the liquid crystal molecules of the liquid crystal layer The second phase plate has a positive refractive index anisotropy and the principal axis of the liquid crystal molecules of the liquid crystal layer. The liquid crystal layer has uniaxial molecules that are twisted in the same direction as the twist direction and the sum of the liquid crystal layer and the liquid crystal twist angle is approximately 360 degrees.This has the following effects. In other words, the combination with the active matrix increases the contrast and improves the viewing angle characteristics. Further, the viewing angle dependence of the liquid crystal layer in the state where no display voltage is applied is broadly divided, and the difference in viewing angle characteristics depending on the azimuth is reduced.
[0032]
  Claims of the invention7The invention described in claim6A liquid crystal twist angle of the liquid crystal layer,Disk-shaped moleculeThe main shaft twist angle,Uniaxial moleculeThe sum of the main shaft twist angle and the sign with one of the twist directions as positive is taken to be -10 degrees or more and within +10 degrees, thereby having the following effects. That is, the consistency of optical propagation characteristics between the liquid crystal layer and the first and second phase plates is enhanced.
[0035]
  Hereinafter, embodiments of the present invention will be described with reference to FIGS.
First embodiment
  FIG. 1 shows a configuration of a liquid crystal display device according to a first embodiment of the present invention. In the figure, reference numerals 4 and 5 denote transparent substrates such as glass substrates, which sandwich the liquid crystal layer 1. The liquid crystal layer 1 has a super twisted nematic (STN) structure with a twist angle of two hundred and ten degrees. Electrodes 2 and 3 for supplying a voltage to the liquid crystal layer 1 are formed on the transparent substrates 4 and 5. 7 and 8 are polarizing plates for performing polarization display, and 9 is a backlight for illumination. Reference numeral 6 denotes a phase plate, which compensates for the wavelength dependence and viewing angle dependence of the optical characteristics of the liquid crystal layer 1 to make the display monochrome and expand the viewing angle characteristics.
[0036]
  2A and 2B are schematic diagrams showing the structure of the liquid crystal layer 1. FIG. 2A shows a state in which no display voltage is applied to the liquid crystal layer 1, and FIG. The display voltage (ON voltage) is applied. The liquid crystal layer 1 has liquid crystal molecules 11 having a positive refractive index anisotropy, and the liquid crystal molecules 11 are in a state where a display voltage is applied (FIG. 2A), the main axis of the liquid crystal molecules is the liquid crystal layer. The liquid crystal molecules are twisted in the thickness direction of the liquid crystal layer 1 in parallel with the surface direction of the liquid crystal layer 1. On the other hand, in the state where the display voltage is applied (FIG. 2B), the liquid crystal molecules 11 are in a state of rising due to the formed electric field, and the twisted arrangement is almost extinguished.
[0037]
  FIG. 3 is a schematic diagram showing the structure of the phase plate 6. The phase plate 6 (corresponding to the first phase plate in the claims) has a uniaxial optical medium 12 having a negative refractive index anisotropy (claim). The optical medium is hereinafter referred to as a negative uniaxial optical medium), the main axis of which is parallel to the surface direction of the phase plate 6, and the main axis is the thickness of the phase plate 6. It has a structure arranged so as to be twisted with respect to the vertical direction. In FIG. 3, for simplification of the drawing, only one row is shown assuming that the negative uniaxial optical medium 12 is arranged in the normal direction, but actually such a structure has the phase plate 6. Spread across the entire surface. Further, each molecule of the negative uniaxial optical medium 12 does not need to have a strict column structure in the normal direction, and statistically has such a twisted structure within a range considered optically homogeneous. Good. As the negative uniaxial optical medium 12, a discotic liquid crystal or other substance having a disc-like molecular structure can be used. Examples of the discotic liquid crystal include a triphenylene compound and a benzene derivative in which long-chain or plate-like functional groups are radially arranged as side chains of the benzene ring. In FIG. 3, the phase plate 6 is formed only from the layer of the uniaxial optical medium 12 having a negative refractive index anisotropy. An optically isotropic support layer (not shown) may be provided on both upper and lower surfaces of the layer of the uniaxial optical medium 12 or on either one of the surfaces.
[0038]
  The liquid crystal layer 1 and the phase plate 6 are twisted by substantially equal angles in opposite directions. The alignment direction of the liquid crystal molecules 11 at the phase plate side interface 1a (see FIG. 1) of the liquid crystal layer 1 and the liquid crystal of the phase plate 6 The side interface 6a (see FIG. 1) is substantially parallel to the principal axis direction of the negative uniaxial optical medium 12. Further, the liquid crystal layer 1 and the phase plate 6 have substantially the same retardation value.
[0039]
  FIG. 4 is a diagram for explaining the principle of improving the viewing angle characteristics by the phase plate 6. The figure shows the molecular arrangement of the liquid crystal layer 1 and the phase plate 6 when no display voltage is applied. For the sake of simplicity, the birefringence (refractive index anisotropy) of the liquid crystal layer 1 and the phase plate 6 is shown. Δn and thickness d are equal, and the principal axes of the liquid crystal molecules 11 and the negative optical medium 12 are parallel to the surface directions of the liquid crystal layer 1 and the phase plate 6. The liquid crystal layer 1 has a left-handed twist and the phase plate 6 has a right-handed twist, and the absolute values of the twist angles are equal.
[0040]
  The liquid crystal layer 1 is divided into thin layers (for example, 10 layers) 21a, 22a,... 30a in order from the phase plate side interface 1a. Similarly, the phase plate 6 is divided into thin layers (same 10 layers as the liquid crystal layer 1) 21b, 22b... 30b in order from the liquid crystal side interface 6a. Considering combinations of thin layers corresponding to the liquid crystal layer 1 and the phase plate 6 such as the thin layers 21a and 21b and the thin layers 22a and 22b after being divided in this way, The combination is a positive / negative birefringent medium having parallel retardation and equal retardation.
[0041]
  Considering the propagation of polarized light in the liquid crystal layer 1 and the phase plate 6 configured as described above, the thin layer 21a and the thin layer 21b are adjacent to each other. The influence of birefringence is canceled out, and when polarization propagation is considered, it can be handled that there is no combination of the thin layer 21a and the thin layer 21b.
[0042]
  Assuming that the combination of the thin layer 21a and the thin layer 21b is not present, the combination of the thin layer 22a and the thin layer 22b can be handled next as a combination. This combination of thin layers is also a positive and negative birefringent medium with parallel main axis directions and equal retardation, so ideally the effects of each other's birefringence will be canceled against light traveling in all directions. When propagation is considered, it can be handled that there is no combination of the thin layer 22a and the thin layer 22b.
[0043]
  In the same way, all combinations from the combination of the thin layer 23a and the thin layer 23b to the combination of the thin layer 30a and the thin layer 30b cancel each other out of viewing angle characteristics, and can be handled as not ideal. it can.
[0044]
  Mathematically, the propagation of polarized light is expressed by the following equation (1) using a propagation matrix T21a, T22a,..., T30a, T21b, T22b,. Is done. Ex and Ey are electric field components having vibration surfaces in the x and y directions, respectively. Hx and Hy are magnetic field components having vibration surfaces in the x and y directions, respectively.
The incident light and the outgoing light are shown.
[0045]
[Expression 1]

  Considering this equation, the above phenomenon becomes a unit matrix in the order of the product of T21a and T21b, the product of T22a and T22b, ... the product of T30a and T30b from the center of the matrix multiplication. And the output polarization are considered to be equal.
[0046]
  If the polarizing plates 7 and 8 are arranged so that their polarization axes are orthogonal to each other in FIG. 1, the optical propagation characteristics of the liquid crystal layer 1 and the phase plate 6 are completely compensated as described above when no display voltage is applied. Therefore, the display is black. This black display is determined by the polarizing plate characteristics and has a very good viewing angle characteristic. As a result, when color display is performed using a mixed color of three pixels in which red (R), green (G), and blue (B) color filters are formed, the color in which no display voltage is applied is observed in the field of view. Even if the angle is shaken, it becomes difficult to leak, the dependence of the hue on the viewing angle is small, and the viewing angle characteristic considering the hue change as well as the contrast becomes good. On the contrary, if the polarizing plates 7 and 8 are arranged so that their polarization axes are parallel to each other, it is possible to perform white display with good viewing angle dependency when no display voltage is applied.
[0047]
  When a display voltage is applied to the liquid crystal layer 1, the liquid crystal molecules 11 that are positive birefringent media rise, and the optical propagation characteristics between the liquid crystal layer 1 and the phase plate 6 deviate from the above compensation conditions. At this time, the value of the twist angle and retardation between the liquid crystal layer 1 and the phase plate 6 and the angle formed between the polarization axis of the polarizing plates 7 and 8 and the principal axis direction of the liquid crystal layer 1 or the phase plate 6 are as follows. Display characteristics can be set as desired.
[0048]
  The above compensation principle is established when the set of thin layers in FIG. 4 is completely symmetric, but in actuality, the alignment direction of the liquid crystal molecules 11 and the negative uniaxial optical medium in the thin layers 21a and 21b. The difference between the 12 main axis directions is about 10 degrees, and the difference in the twist angle between the liquid crystal layer 1 and the phase plate 6 is 20 degrees or less (that is, considering the sign with one of the twist directions being positive) If the sum of the twist angles of the two is within a range of −20 degrees to +20 degrees), a viewing angle characteristic that is practically sufficient can be obtained by adjusting the arrangement angles of the polarizing plates 7 and 8 slightly. When viewed from the front, it can be displayed without causing black coloring.
[0049]
  In addition, when defining the twist angle and the arrangement angle of each optical element, it shall be defined by the angle when viewed from the normal direction of the liquid crystal layer 1 and the phase plate 6, that is, the orthogonal projection to the interface, The inclination angle is not considered.
[0050]
  Next, the reason why the birefringence effect between the liquid crystal layer 1 and the phase plate 6 is canceled by providing the phase plate 6 having the negative uniaxial optical medium 12 will be described with reference to FIG. .
[0051]
  FIG. 5A shows a model configuration of a liquid crystal display device in which a phase plate (hereinafter, such a phase plate is referred to as a positive phase plate) 52 having a positive uniaxial optical medium 62 is stacked on the liquid crystal layer 1 ′. FIG. 5 (b) shows an application in which a phase plate (hereinafter referred to as a negative phase plate) 6 ′ having a negative uniaxial optical medium 12 ′ is laminated on a similar liquid crystal layer 1 ′. It is sectional drawing which shows the model structure of the liquid crystal display device of invention. In FIG. 5, in order to simplify the description, the liquid crystal molecules 11 ′, the positive uniaxial optical medium 62, and the negative uniaxial optical medium 12 ′ are arranged with the main axes of the liquid crystal layer 1 ′ and the phase plate 6 ′. It is arranged in parallel with the surface direction.
[0052]
  The liquid crystal molecules 11 'are arranged in parallel with the main axis thereof in the left-right direction in the figure. The liquid crystal molecules 11 ′ have optical characteristics as a positive uniaxial optical medium. That is, neLC> noLC between the extraordinary refractive index neLC, which is the refractive index for light oscillating in the axial direction, and the ordinary refractive index noLC, which is the refractive index for light oscillating in a direction perpendicular to the axial direction. There is a relationship.
[0053]
  The positive phase plate 52 is arranged so that the optical axis direction of the positive uniaxial optical medium 62 is orthogonal to the liquid crystal molecules 11 ′. That is, the positive uniaxial optical media 62 are arranged in parallel with the main axis thereof in a direction perpendicular to the paper surface. The positive uniaxial optical medium 62 has optical characteristics as a positive uniaxial optical medium. That is, there is a relationship ne1> no1 between the extraordinary refractive index ne1 and the ordinary refractive index no1.
[0054]
  The negative phase plate 6 ′ is arranged so that the negative uniaxial optical medium 12 ′ is parallel to the liquid crystal molecules 11 ′. That is, the negative uniaxial optical medium 12 'is arranged in parallel with its main axis as the left-right direction in the figure. The negative uniaxial optical medium 12 ′ has optical characteristics as a negative uniaxial optical medium. That is, there is a relationship ne2 <no2 between the extraordinary refractive index ne2 and the ordinary refractive index no2.
[0055]
  Next, consider the case where the vertical light 56 is incident on the configuration shown in FIGS. When the vertical light 56 passes through the liquid crystal layer 1 ′, a phase difference is generated between the polarization component 56 </ b> V having a polarization plane in the direction perpendicular to the paper surface and the polarization component 56 </ b> H having a polarization surface inward in the paper surface. In both the configuration of 5 (a) and the configuration of FIG. 5 (b), if the retardation of the liquid crystal layer 1 ′ is equal to the retardation of the phase plates 52 and 6 ′, the liquid crystal layer 1 ′ passes. The phase difference is canceled by the phase plates 52 and 6 ′ to cancel the phase difference generated in the liquid crystal layer 1 ′, thereby creating a state of zero phase difference as a whole.
[0056]
  As described above, in the vertical light 56, the light emitted through the phase plates 52 and 6 ′ has a phase difference of zero regardless of the positive phase plate 52 or the negative phase plate 6 ′.
[0057]
  Next, consider a case in which inclined light 57 propagating in a direction inclined by an angle (incident angle) α from the vertical direction is incident on the liquid crystal layer 1 ′ and the phase plates 52 and 6 ′.
[0058]
  The distance that the inclined light 57 passes through the liquid crystal layer 1 ′ and the phase plates 52, 6 ′ is longer than the vertical light 56. For example, as shown in FIG. 5B, when the thickness of the negative phase plate 6 ′ is d, the distance that the vertical light 57 passes through the negative phase plate 6 ′ is d / cos α. The same applies to the liquid crystal layer 1 ′ and the positive phase plate 62.
[0059]
  In view of optical anisotropy, the liquid crystal molecules 11 ′ and the negative uniaxial optical medium 12 ′ propagate light in a direction closer to the optical axis as the incident angle α increases. Therefore, the refractive index anisotropy between the two polarization components 57V and 57H of the tilted light 57 is reduced by a factor of COS2α.
[0060]
  In general, since the retardation can be regarded as a product of refractive index anisotropy and propagation distance, in the negative phase plate 6 ′, the retardation of the liquid crystal layer 1 ′ and the retardation of the negative phase plate 6 ′ are: As the incident angle α increases, both decrease from the retardation of the vertical light 56 at a rate of cos α.
[0061]
  On the other hand, in the positive phase plate 52, the long axis of the positive uniaxial optical medium 62 and the light propagation direction are orthogonal to each other in the vertical light 56 and the inclined light 57. The refractive index anisotropy of these two polarization components 56V and 56H (57V and 57H) does not vary with a change in the angle α. Therefore, in the calculation of the retardation variation, only the distance factor remains, and the retardation of the positive phase plate 52 increases from the retardation in the vertical direction at a rate of 1 / cos α.
[0062]
  Therefore, if the tilted light 57 is incident on the configuration of FIG. 5A having the positive phase plate 52, the retardation of the liquid crystal layer 1 ′ decreases while the retardation of the positive phase plate 52 increases. However, the compensation relationship between the two will collapse.
[0063]
  On the other hand, if the tilted light 57 is incident on the configuration of FIG. 5B having the negative phase plate 6 ′, the retardation of the liquid crystal layer 1 ′ and the retardation of the negative phase plate 6 ′ are obtained. Since both decrease by approximately the same amount, the compensation relationship between the two is kept good.
[0064]
  In the above description, the case where the incident angle α of light is tilted from the vertical direction to the right and left in the drawing is applicable. On the other hand, when the incident angle α of light is inclined in the depth direction or the near side of the paper, the liquid crystal layer 1 ′ and the negative phase plate 6 ′ have a refractive index anisotropy even if the incident angle α increases. Is constant, the retardation increases as the incident angle α increases. On the other hand, in the positive phase plate 52, the refractive index anisotropy decreases with a factor of cos 2α as the incident angle α increases, so that the retardation decreases.
[0065]
  Thus, even when the incident angle α of the light is inclined in the depth direction or the near side of the paper surface, the configuration of FIG. 5A including the positive phase plate 52 compensates for the inclined light 57. It can be seen that, while the characteristics deteriorate, in the configuration of FIG. 5B provided with the negative phase plate 6 ′, the compensation characteristics for the inclined light 57 are kept good. When the incident angle α is at an intermediate angle position between the horizontal direction and the vertical direction, the retardation at that time is the retardation in the state of FIG. 5A and the retardation in the state of FIG. 5B. An intermediate value between the base and the base is shown. In addition, in order to simplify description, in the above description, refraction of light at each interface is omitted.
[0066]
  Thus, when the negative phase plate 6 ′ is laminated on the liquid crystal layer 1 ′, the viewing angle dependency of both retardations is in the same direction, and the viewing angle dependency is higher than that of the liquid crystal layer 1 ′ alone. Will be alleviated. Therefore, as long as the negative phase plate 6 ′ is mounted, even if the retardation of the liquid crystal layer 1 ′ and the negative phase plate 6 ′ is not necessarily equal, the configuration of the liquid crystal layer 1 ′ alone or the liquid crystal layer 1 ′ Compared with the configuration in which the positive phase plate 52 is laminated, the viewing angle characteristics of the liquid crystal display device can be improved. If the retardations of the liquid crystal layer 1 ′ and the negative phase plate 6 ′ are equal to or in accordance with each other, the viewing angle characteristics are further improved.
[0067]
  In the above description, in order to simplify the description, it is assumed that both the liquid crystal layer 1 ′ and each of the phase plates 52 and 6 ′ do not have a twisted structure, but this principle also applies to the case where there is a twisted structure. In particular, as shown in FIG. 4 and the like, when the liquid crystal layer 1 and the phase plate 6 can be divided into several thin layers and optical compensation in each combination can be considered, each thin layer has a twist angle. Since it can be considered to be small and approximate to a parallel arrangement, the above-described explanation can be applied as it is to the combination of each thin layer.
[0068]
  By the way, an actual STN type liquid crystal display device is almost always driven by a duty. In this case, a slight voltage (off voltage) is applied to the liquid crystal layer 1 even in the off state, and the liquid crystal molecules 11 are slightly up. Therefore, the retardation of the liquid crystal layer 11 as viewed from the front direction is slightly smaller than the state where no display voltage is applied. Therefore, it is desirable to set the retardation of the phase plate 6 slightly smaller than the retardation of the liquid crystal layer 1 in order to obtain better display characteristics in the front direction.
[0069]
  FIG. 6 shows experimental results relating to the relationship between the ratio Rn / RLC between the retardation Rn of the phase plate 6 and the retardation RLC of the liquid crystal layer 1 and the display contrast. The conditions of this experiment are as follows. That is, the drive duty ratio of the liquid crystal display device is 1/300, the retardation of the liquid crystal layer 1 is 840 nm, the twist angle of the liquid crystal layer 1 is 250 degrees counterclockwise, and the twist angle of the phase plate 6 is 250 degrees clockwise. The angle between the polarization axis direction of the polarizing plate 8 and the alignment direction of the liquid crystal molecules on the lower surface of the liquid crystal layer 1 is 45 degrees, and the polarization axis of the polarizing plate 7 and the polarization axis of the polarizing plate 8 are orthogonal.
[0070]
  As is clear from the experimental results, in order to obtain a relatively high contrast of 20 or more, it is desirable to set Rn / RLC between 0.85 and 0.96. Furthermore, in order to obtain a high contrast of 40 or more, it is desirable to set Rn / RLC between 0.88 and 0.95.
[0071]
  FIG. 7 shows the viewing angle dependence of contrast when the above ratio (Rn / RLC) is 0.92. FIG. 7 shows isocontrast curves up to a tilt angle of 80 degrees with respect to all azimuth angles. FIG. 8 shows a phase plate in which a uniaxial optical medium having a positive refractive index anisotropy is twisted and arranged under the same conditions. At the liquid crystal layer side interface, the main axis of the optical medium is the orientation direction of liquid crystal molecules. FIG. 8 shows the viewing angle dependence of contrast when arranged so as to be orthogonal to each other.
[0072]
  In these figures, a region i indicated by a diagonal line rising to the right in the periphery indicates that the contrast is 1 or less and the display is reversed. However, a uniaxial optical medium having negative refractive index anisotropy is provided. The characteristics shown in FIG. 7 are greatly reduced in the display inversion area (a) as compared with the characteristics shown in FIG. 8 in which a uniaxial optical medium having positive refractive index anisotropy is provided. Further, a region B having a contrast of 20 or more indicated by a left-upward oblique line in the center is wider in FIG. That is, it can be seen that the viewing angle characteristics are greatly improved by using a uniaxial optical medium having a negative refractive index anisotropy and having a twist direction opposite to that of the liquid crystal layer 1.
[0073]
  In order to perform duty driving, the luminance-voltage characteristic needs to be steep, but in order to obtain this steepness, the twist angle of the liquid crystal molecules in the liquid crystal layer 1 is desirably 200 degrees or more, and 240 degrees If it is above, a more favorable display characteristic is acquired. On the other hand, if the twist angle is too large, an orientation failure phenomenon is likely to occur. Therefore, the twist angle is preferably set to 300 degrees or less, more preferably 270 degrees or less.
[0074]
  Further, the angle formed between the polarization axis of the polarizing plate 8 and the alignment direction of the liquid crystal molecules at the polarizing plate side interface 1a of the liquid crystal layer 1 is 15 degrees in the sense of obtaining steepness of the characteristics as described above. It is desirable to set in the range of ˜75 degrees, more desirably in the range of 30 degrees to 60 degrees. If the twist axis of the liquid crystal layer 1 and the phase plate 6 are equal, the polarization axis direction of the polarizing plate 7 may be substantially orthogonal to the polarization axis direction of the polarizer plate 8. However, it is only necessary to rotate it by that amount, but if it is adjusted in the range of several degrees to several tens of degrees from each of the above-mentioned directions, better display characteristics can be obtained. In order to improve the viewing angle characteristics of white display, the polarization axes of the polarizing plates 7 and 8 are adjusted in the parallel direction or in the range of several degrees or several tens of degrees from the direction in which the difference of the twist angle is added. Is desirable.
[0075]
  When the retardation of the liquid crystal layer 1 is too small, the ON luminance is insufficient, and when it is too large, the ON display is colored yellowish. Therefore, when performing monochrome display or three-color mixed color display using RGB color filters, the retardation of the liquid crystal layer 1 is preferably set between 700 and 950 nm, and more preferably between 780 and 900 nm. Display characteristics can be obtained. In particular, when the twist angle is around 250 degrees, if the retardation of the liquid crystal layer 1 is set to around 840 nm, very good display characteristics can be obtained.
Second embodiment
  The liquid crystal display device of the present embodiment has basically the same configuration as the liquid crystal display device of the first embodiment. The main difference from the first embodiment is the configuration of the phase plate. In the first embodiment, the retardation of the phase plate 6 is set slightly smaller than the retardation of the liquid crystal layer 1 in order to improve the display characteristics from the front during duty driving. In contrast, the phase plate 14 of the present embodiment tilts the main axis of the negative uniaxial optical medium 15 from the surface direction of the phase plate 14 upward to the right in the drawing by an angle θ as shown in FIG. In addition, the twisted arrangement improves the front characteristics during duty driving. In FIG. 9, the principal axis tilt angle θ of the negative uniaxial optical medium 15 is constant regardless of the position z in the depth direction of the phase plate 14 of the negative uniaxial optical medium 15, but this has a slight distribution. You may have it.
[0076]
  The tilt angle of the principal axis at the position z in the depth direction of the phase plate 14 is θz, the intrinsic birefringence is Δn, and the thickness of the phase plate is d. In this case, the equivalent retardation Reff from the front direction of the phase plate 14 is expressed by the following equation (2).
[0077]
[Expression 2]

  As in the first embodiment, the ratio Reff / RLC between the equivalent retardation Reff from the front direction of the phase plate 14 and the retardation RLC when the display voltage is not applied to the liquid crystal layer 1 is 0. Good display characteristics can be obtained by setting between 85 and 0.96, more preferably between 0.88 and 0.95.
[0078]
  Note that when the negative uniaxial optical medium 15 does not have the above-described tilt structure, the value of the retardation Reff coincides with the normal retardation. When it is difficult to determine the tilt angle θ and the value of the retardation Reff cannot be obtained from the above equation (2), the polarized light is incident on the phase plate 14 and the state of the output polarized light is observed. Compared with the theoretical output polarization state obtained by simulation, the equivalent retardation value can be obtained relatively easily.
[0079]
  The direction of the tilt angle θ is such that the main axis of the negative uniaxial optical medium 15 at the liquid crystal side interface of the phase plate 14 and the main axis of liquid crystal molecules at the phase plate side interface 1a of the liquid crystal layer 1 are in the same direction. That is, in order to increase the viewing angle, it is desirable to set the main axes of the thin layer 21a and the thin layer 21b to be inclined in the same direction in FIG. The twist angle and retardation value of the liquid crystal layer 1 and the arrangement of the polarizing plates 7 and 8 may be set in the same manner as in the first embodiment.
[0080]
  In the present embodiment, the main axis of the negative uniaxial medium 15 in the phase plate 14 is tilted (tilted) in a direction slightly rising from the surface direction of the phase plate 14, so that the first embodiment In contrast, since a slight off-voltage is applied even in the off-display state, the optical alignment between the liquid crystal layer 1 in which the liquid crystal molecules rise slightly and the phase plate 14 is improved, and the compensation characteristic for the viewing angle is improved. improves.
Third embodiment
  The liquid crystal display device of the present embodiment has basically the same configuration as the liquid crystal display devices of the first and second embodiments. The main difference from the first and second embodiments is the configuration of the phase plate. In the present embodiment, the idea of the second embodiment is further advanced, and the following phase plate 16 is provided. That is, the phase plate 16 has a negative uniaxial optical medium 17 whose tilt angle has a distribution in the depth direction corresponding to the degree of tilt of the liquid crystal molecules 11.
[0081]
  In the simple matrix type liquid crystal display device, a slight voltage (hereinafter, this voltage is referred to as an off voltage) is applied to the liquid crystal layer 1 even in the off display state due to its structure. In a state where such an off-voltage is applied to the liquid crystal layer 1, the liquid crystal molecules 11 hardly move from the initial alignment state due to a so-called anchoring phenomenon at the interface between the liquid crystal layer 1 and the transparent substrates 4 and 5. In the vicinity of the central portion in the thickness direction of the layer 1, the liquid crystal molecules 11 move with a change in voltage.
[0082]
  When an off voltage is applied, the alignment direction of the liquid crystal molecules corresponding to the position z in the depth direction of the liquid crystal layer 1 can be obtained by simulation. FIG. 10 shows the calculation results of the tilt angle θ and the twist (twist) angle φ of the liquid crystal molecules with respect to each position z in the liquid crystal thickness direction when the off voltage is applied. In this calculation, the pretilt angle is 6 degrees.
[0083]
  In the present embodiment, the tilt inclination of the main axis of the negative uniaxial optical medium 17 included in the phase plate 16 is distributed as shown in FIG. 11 corresponding to the calculation result. That is, with the twist angle of the negative uniaxial optical medium 17 opposite to the twist angle of the liquid crystal molecules, the distribution of the tilt angle of the negative uniaxial optical medium 17 in the thickness direction is applied when an off voltage is applied ( The tilt angle distribution of the liquid crystal molecules in the thickness direction when no display voltage is applied is matched.
[0084]
  The arrangement relationship between the liquid crystal molecules 11 and the negative uniaxial optical medium 17 of the phase plate 16 in the present embodiment is as shown in FIG. FIG. 12 is a diagram according to FIG. 4, and the same parts are denoted by the same reference numerals.
[0085]
  Also in FIG. 12, the main axis of the liquid crystal molecule 11 and the negative uniaxial optical medium in each combination of the thin layer 21a and the thin layer 21b, the thin layer 22a and the thin layer 22b,... The 17 main axes are parallel to each other. Therefore, on the same principle as described in the first embodiment, each combination of thin layers can cancel each other's optical propagation characteristics more satisfactorily, and a wide viewing angle characteristic can be obtained.
[0086]
  In this embodiment, since the negative uniaxial optical medium of the phase plate 16 is arranged so as to compensate the optical characteristics when the off voltage is applied to the liquid crystal layer 1, the off voltage is applied even in the off display state. In the structure to be achieved, better viewing angle characteristics can be obtained as compared with the first and second embodiments.
[0087]
  In the first, second, and third embodiments, the phase plates 6, 14, and 16 are on the front side when viewed from the liquid crystal layer 1. In FIG. A light 9 was arranged. However, the backlight 9 may be disposed behind the polarizing plate 7 and the display may be observed from the polarizing plate 8 side. In this case as well, a good viewing angle characteristic can be displayed in the same manner as described above.
Fourth embodiment
  FIG. 13 shows a configuration of a liquid crystal display device according to a fourth embodiment of the present invention. In the present embodiment, a phase plate having a negative uniaxial optical medium is divided into a first phase plate 6A and a second phase plate 6B, and these phase plates 6A and 6B are separated from both sides of the liquid crystal layer 1. It is characterized by having been arranged in.
[0088]
  In the present embodiment, liquid crystal molecules (not shown) at the first phase plate side interface 1b of the liquid crystal layer 1 and a negative uniaxial optical medium (not shown) at the liquid crystal layer side interface 6b of the first phase plate 6A. The angle formed by the main axis of) should theoretically be parallel. Similarly, the liquid crystal molecules (not shown) at the second phase plate side interface 1c of the liquid crystal layer 1 and the main axis of the negative uniaxial optical medium (not shown) at the liquid crystal layer side interface 6c of the second phase plate 6B. The angle between the two should theoretically be parallel. However, as explained in the first embodiment, practical viewing angle characteristics can be obtained if each angle described above is practically 10 degrees or less.
[0089]
  As described in the first embodiment, the principle of optical compensation is that the liquid crystal layer 1 and the first and second phase plates 6A and 6B are divided into thin layers, and the liquid crystal layer 1 and the phase plates 6A and 6B are divided. It can be considered that the combination of the thin layers with each other cancels the optical characteristics sequentially.
[0090]
  Furthermore, in the configuration of the present embodiment, for example, when the retardations of the first and second phase plates 6A and 6B are set to be equal, the first half of the characteristic of the upper half of the liquid crystal layer 1 is positioned on the upper side. The phase plate 6A compensates, and the second phase plate 6B located on the lower side compensates the lower half of the characteristics of the liquid crystal layer 1. As a result, the thickness of the liquid crystal layer 1 compensated by each of the phase plates 6A and 6B is only half that of the configuration of the first embodiment. Therefore, when considering optical compensation with a combination of thin layers, if there is a mismatch in compensation characteristics, that is, the matching of the optical characteristics of the corresponding thin layers between the liquid crystal layer 1 and the phase plates 6A and 6B. Even if there is a discrepancy, the effect is difficult to accumulate, resulting in good viewing angle characteristics. Further, since the first phase plate 6A and the second phase plate 6B are provided on the upper and lower sides of the liquid crystal layer 1, there is an advantage that the symmetry of the configuration is increased and the viewing angle characteristics are more symmetric. Even if the retardations of the first and second phase plates 6A and 6B are not necessarily equal, the effects described here are exhibited.
[0091]
  In the first embodiment, the ratio Rn / RLC between the retardation Rn of the phase plate 6 and the retardation RLC of the liquid crystal layer 1 is preferably set between 0.85 and 0.96, and 0.88. Although it has been described that it is more desirable to set between ˜0.95, this embodiment is as follows. That is, the ratio (Rn1 + Rn2) / RLC of the sum (Rn1 + Rn2) of the retardation Rn1 of the first phase plate 6A and the retardation Rn2 of the second phase plate 6B to the retardation RLC of the liquid crystal layer 1 is 0.85. It is preferably between 0.96 and more preferably between 0.88 and 0.95.
[0092]
  As for the twist angle, the total sum of the twist angles of the liquid crystal layer 1 and the first and second phase plates 6A and 6B in consideration of the sign with one twist direction being positive is -20 degrees to +20. If it is within the range of degrees, a practically sufficient viewing angle characteristic can be obtained.
[0093]
  Regarding the arrangement angle of the polarizing plate, the angle formed between the polarizing axis of the polarizing plate 8 and the main axis of the negative uniaxial optical medium (not shown) at the polarizing plate 8 side interface 6d of the second phase plate 6B. Is preferably set in the range of 15 to 75 degrees, more preferably in the range of 30 to 60 degrees, the liquid crystal characteristics suitable for multiplex driving can be obtained with a steep luminance-voltage characteristic.
[0094]
  In FIG. 14, the phase plate 6 used in the first embodiment is divided into two phase plates 6A and 6B having a twist angle of 125 degrees and equal retardation, and arranged on both sides of the liquid crystal layer 1. The viewing angle dependence of the contrast of the liquid crystal display device of 4th Embodiment is shown.
[0095]
  As is apparent from this figure, the viewing angle dependency of the liquid crystal display device of the present embodiment (see FIG. 14) compared to the viewing angle dependency of the liquid crystal display device of the first embodiment (see FIG. 7). In this case, the inclination range with a contrast of 50 or more is increased, and the left-right symmetry of the viewing angle characteristic is improved. For example, in the upper part of the figure, an area having a contrast of 50 or more (indicated by reference symbol C in FIGS. 7 and 14) is enlarged by about 10 degrees at a tilt angle, and in the figure, an area having a contrast of 20 or more in the upper right and lower right directions. B is about 10 degrees larger than the fall angle. As a result, in the liquid crystal display device of the present embodiment, it is possible to obtain a display that is symmetric and has a wide viewing angle and is very easy to see.
[0096]
  When the liquid crystal display device of the present embodiment is desired to be an NB display mode with good viewing angle dependency of black display, the twist axis of the polarizing plate 7 is twisted from the direction perpendicular to the polarizing axis of the polarizing plate 8. What is necessary is just to arrange | position in the direction which only the part for the sum total of a corner. At this time, if the adjustment is made in the range of several degrees to several tens of degrees from the above direction, still better display characteristics can be obtained. In addition, when the NW display mode in which the viewing angle characteristic of white display is favorable is desired, the polarization axis of the polarizing plate 7 may be considered in the same manner with reference to the direction parallel to the polarizing axis of the polarizing plate 8.
[0097]
  In the liquid crystal display device in which negative phase plates (having uniaxial optical media having negative refractive index anisotropy) 6A and 6B are arranged on both sides of the liquid crystal layer 1 as in the present embodiment, As shown in the second embodiment, the viewing angle characteristics can be further improved by inclining the principal axis of the negative uniaxial optical medium of each of the phase plates 6A and 6B. Similarly, as shown in the third embodiment, the negative uniaxial optical media of the phase plates 6A and 6B are arranged in accordance with the arrangement state of the liquid crystal molecules, thereby further improving the viewing angle characteristics. You can also
Fifth embodiment
  FIG. 15 shows the configuration of a liquid crystal display device according to the fifth embodiment of the present invention. In the figure, the same or equivalent parts as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted.
[0098]
  The liquid crystal display device according to the present embodiment is characterized by a positive refractive index difference between the liquid crystal layer 1 and a phase plate (negative phase plate) 6 made of a uniaxial optical medium having negative refractive index anisotropy. A phase plate (positive phase plate: corresponding to the second phase plate in the claims) 18 provided with a uniaxial optical medium 19 having directionality is further provided.
[0099]
  FIG. 16 schematically shows the arrangement of positive and negative uniaxial optical media 19 and 12 in the liquid crystal layer 1 and the two phase plates 18 and 6. In this figure, components such as the transparent substrates 4 and 5 are not shown. The phase plate 10 has a positive uniaxial optical medium 19 having positive refractive index anisotropy, and the positive uniaxial optical medium 19 is twisted in the same direction as the liquid crystal layer 1. The principal axis direction of the positive uniaxial optical medium 19 at the liquid crystal layer side interface 18 a of the phase plate 18 substantially coincides with the principal axis direction of the liquid crystal molecules 11 at the phase plate 18 side interface 1 d of the liquid crystal layer 1. The twist direction of the positive uniaxial optical medium 19 is the same as the twist direction of the liquid crystal molecules 11, and the twist angle of the liquid crystal molecules 11 in the liquid crystal layer 1 and the twist angle of the positive uniaxial optical medium 19 in the phase plate 18. The total is 360 degrees.
[0100]
  As in the first embodiment, the phase plate 6 has a negative uniaxial optical medium 12 having negative refractive index anisotropy and twisted in the opposite direction to the liquid crystal layer 1. ing.
[0101]
  Also in the present embodiment, the phase plate 6 compensates for the viewing angle dependence caused by the liquid crystal layer 1 and the phase plate 18 on the same principle as the first embodiment, so that a liquid crystal display device having a very wide viewing angle can be obtained. Obtainable. As described in the first embodiment, this is because the phase plates 6 and 18 and the liquid crystal layer 1 are divided into several thin layers, and the combination of thin layers having positive and negative refractive index anisotropy is mutually connected. It can be thought of as compensating for the optical properties. In an ideal state, complete optical compensation is performed in all directions. However, in reality, the value of the refractive index and the orientation of the principal axes of the liquid crystal molecules 11 and the uniaxial optical media 12 and 19 are not necessarily ideal. Therefore, when the light traveling direction is in the vicinity of the axial direction (the normal direction of the phase plates 6 and 18 and the liquid crystal layer 1) in the combination of the thin layers, the compensation characteristic becomes good, and then the light traveling direction deviates. Compensation is often incomplete.
[0102]
  However, since this embodiment is provided with the phase plate 18 having the positive uniaxial optical medium 19 as compared with the first embodiment, the uniaxial optical medium or liquid crystal in the combination of the thin layers as described above. The direction of the principal axis of the molecule exists for all directions of 360 degrees, the axial direction dependency in the viewing angle compensation is dispersed in all directions, and a better viewing angle characteristic can be obtained.
[0103]
  The above explanation is as follows from a different viewpoint. That is, the phase plate 18 having the positive uniaxial optical medium 19 relaxes or averages the azimuth angle dependence in the viewing angle characteristics of the liquid crystal display, and the phase plate 6 having the negative uniaxial optical medium compensates for this viewing angle dependence. It is considered to have a function.
[0104]
  Since this compensation is particularly effective in the display voltage non-application state (off state) as in the first embodiment, the polarization axes of the polarizing plates 7 and 8 are arranged almost orthogonally (NB display). Thus, a black display with a very good viewing angle can be obtained, and a white display with a very good viewing angle can be obtained if the polarizing axes of the polarizing plates 7 and 8 are arranged substantially parallel (NW display). Can do.
[0105]
  In the configuration of the present embodiment, the relationship between the twist angles of the liquid crystal molecules 11 and the positive and negative uniaxial optical media 12 and 18 is preferably in the range of the following formula (3), and further, the formula (4) It is further desirable to be in range.
[0106]
                    -30 ° ≦ ΩLC + Ωp + Ωn ≦ 30 °… (3)
                    -10 ° ≦ ΩLC + Ωp + Ωn ≦ 10 °… (4)
     ΩLC: Twist angle of liquid crystal molecule 11
      Ωp: twist angle of positive uniaxial optical medium 19
      Ωn: twist angle of negative uniaxial optical medium 12
  Note that the twist direction is labeled with a left twist being positive and a right twist being negative.
[0107]
  In the present embodiment, the twist angle of the liquid crystal layer 1, the angle formed between the polarization axis of the polarizing plate 8 and the liquid crystal molecules 11, and the polarization axis of the polarizing plate 7 and the polarization axis of the polarizing plate 8 About the angle formed between, it is desirable to set to the range similar to 1st Embodiment.
[0108]
  In the proximity surface between the liquid crystal layer 1 and each of the phase plates 6 and 18, the angle between the orientation direction and the principal axis direction of the liquid crystal molecules 11 and the positive and negative uniaxial optical media 19 and 12, that is, the liquid crystal layer 1 in FIG. The angle formed between the orientation direction of the liquid crystal molecules 11 at the phase plate side interface 1d and the principal axis direction of the positive uniaxial optical medium 19 at the liquid crystal layer side interface 18a of the phase plate 18, and the phase plate 6 of the phase plate 18 The angle formed between the main axis direction of the positive uniaxial optical medium 19 at the side interface 18b and the main axis direction of the negative uniaxial optical medium 12 at the liquid crystal layer side interface 6e of the phase plate 6 will be described.
[0109]
  In other words, considering the state where no voltage is applied to the liquid crystal layer 1, the ideal state is that these angles are theoretically parallel. However, in reality, if both of these angles are within 10 degrees, a viewing angle characteristic in a state in which no display voltage is applied is obtained sufficiently practically. Therefore, other characteristics such as luminance characteristics and hue in a state in which a display voltage is applied are obtained. If these angles are adjusted so that the display characteristics are taken into consideration and the display voltage is not applied, the overall display characteristics can be further improved.
[0110]
  The phase plate 18 having the positive uniaxial optical medium 19 preferably has a structure that is regarded as an extension of the liquid crystal layer 1. Therefore, the retardation Rp of the phase plate 18 may be considered based on the value Rpo calculated by the following equation (5).
[0111]
       Rpo = RLC x Ωp / ΩLC (5)
          RLC: retardation of liquid crystal layer 1
         Ωp: twist angle of positive uniaxial optical medium 19
           ΩLC: Twist angle of liquid crystal molecule 11
However, considering that the liquid crystal display device is duty-driven, if the retardation of the phase plate 18 is set to a value about 10% smaller than this, or if the positive uniaxial optical medium 19 is inclined, Further, the optical continuity with the liquid crystal layer 1 is improved. When the positive uniaxial optical medium 19 is tilted, it is desirable that the direction of the tilted alignment be the same as the tilt direction of the liquid crystal molecules 11.
[0112]
  The equivalent retardation Rpeff of the phase plate 18 considering the inclined structure is a value calculated by the following equation (6) in the same manner as in the first embodiment.
[0113]
[Equation 3]

               Δnp: intrinsic birefringence of the positive uniaxial optical medium 19
  If the phase plate 18 is not tilted, the equivalent retardation Rpeff of the phase plate 18 is equal to the retardation Rp selected based on Rpo calculated by the above equation (5).
[0114]
  In the configuration of the present embodiment, a part of the retardation of the phase plate 6 having the negative uniaxial optical medium 12 compensates for the optical characteristics of the phase plate 18 having the positive uniaxial optical medium 19, and the phase plate 6 It is considered that the remaining part of the retardation compensates for the optical characteristics of the liquid crystal layer 1. Therefore, the portion of the retardation of the phase plate 6 that compensates for the optical characteristics of the liquid crystal layer 1 is Rn−Rpeff.
[0115]
  As described in the first embodiment, since the liquid crystal layer 1 is duty-driven, the first is between Rn−Rpeff, which is a part for compensating the optical characteristics of the liquid crystal layer 1, and the retardation of the liquid crystal layer 1. It is desirable that the relationship described in the embodiment is satisfied. That is, in order to realize a good viewing angle characteristic, it is desirable that the following expression (7) is satisfied. Furthermore, if the following expression (8) is satisfied, a better viewing angle characteristic is obtained. Obtainable.
[0116]
              0.85 ≦ (Rn−Rpeff) /RLC≦0.96 (7)
              0.88 ≦ (Rn−Rpeff) /RLC≦0.95 (8)
  FIG. 17 shows the viewing angle dependence of contrast in the liquid crystal display device of the present embodiment. Compared with the viewing angle characteristics of the first embodiment shown in FIG. 7, there is almost no difference in the display inversion area, but the area B with a contrast of 20 or more and the area C with a contrast of 50 or more spread in the lower left direction in the figure. It can be seen that the viewing angle characteristics are improved.
Modification 1 of the fifth embodiment
  In the liquid crystal display device of the fifth embodiment, as in the second embodiment, the phase plate 6 having the negative uniaxial optical medium has the negative uniaxial optical medium 15 whose principal axis is tilt-aligned. A phase plate similar to the phase plate 14 may be used. Hereinafter, such a phase plate is referred to as a phase plate 6 (14).
[0117]
  If it does so, the front characteristic at the time of a duty drive will become favorable on the principle similar to description in 2nd Embodiment. That is, the equivalent retardation Rneff from the front direction of the phase plate 6 (14) is equal to the value of Reff obtained by the above-described equation (2). Further, in the above description of the fifth embodiment, it has been described that it is desirable to set the value of the retardation Rn within the range indicated by the above-described equations (7) and (8), but the phase plate 6 (14 ), The retardation Rn may be replaced with the retardation Rneff.
[0118]
  The tilt direction of the negative uniaxial optical medium included in the phase plate 6 (14) may be set as follows. That is, the tilt structure of the liquid crystal layer 1 is virtually extended into the positive phase plate. The tilt of the negative uniaxial optical medium included in the phase plate 6 (14) is such that the tilt direction of the main axis of the negative uniaxial optical medium in the phase plate 6 (14) is continuous with the virtual tilt structure. It is desirable to set the direction.
[0119]
  When the phase plate 6 (14) is provided, the main axis of the negative uniaxial optical medium included in the phase plate 6 (14) is inclined in a direction slightly rising from the surface direction of the phase plate 6 (14). Compared with the structure of the fifth embodiment having a normal phase plate 6, the optical matching between the liquid crystal layer 1 in which the liquid crystal molecules 11 slightly rise due to the off voltage and the phase plate 6 (14) is better. Thus, the compensation characteristic for the viewing angle is further improved.
Modification 2 of the fifth embodiment
  In the liquid crystal display device of the fifth embodiment, as the phase plate 6 having the negative uniaxial optical medium 12, the phase plate 16 having the negative uniaxial optical medium 17 shown in the third embodiment and A similar phase plate may be used. Hereinafter, such a phase plate is referred to as a phase plate 6 (16), and a negative uniaxial optical medium included in the phase plate 6 (16) is referred to as a negative uniaxial optical medium 12 (17).
[0120]
  The negative uniaxial optical medium 12 (17) has the following configuration. That is, the tilt angle of the negative uniaxial optical medium 12 (17) has a predetermined distribution along the depth direction of the phase plate 6 (16). This predetermined distribution corresponds to the tilt of the liquid crystal molecules 11 of the liquid crystal layer 1 and the tilt of the main axis of the positive uniaxial optical medium 19 of the positive phase plate 18.
[0121]
  The alignment distribution of the liquid crystal molecules 11 when an off voltage is applied to the liquid crystal layer 1 can be obtained by simulation in the same manner as in the third embodiment. In the present embodiment, in addition to this, the main axis of the positive uniaxial optical medium 19 is also taken into consideration, and the tilt angle of the main axis of the negative uniaxial optical medium 12 (17) is correspondingly set to the phase plate 6 (16 ) Distributed in the thickness direction.
[0122]
  In the second modification, by distributing the tilt angles in the principal axis direction of the negative uniaxial optical medium 12 (17) as described above, the liquid crystal layer 1 is the same as described in the third embodiment. The optical characteristics when the off voltage is applied to are more faithfully compensated. Therefore, a better viewing angle characteristic can be obtained as compared with the fifth embodiment.
Modification 3 of the fifth embodiment
  In the fifth embodiment described above and the first and second modifications thereof, either one or both of the positive and negative phase plates 18 and 6 are divided into two phase plates and arranged one by one above and below the liquid crystal layer 1. As a result, even better display characteristics can be obtained.
[0123]
  The reason why good display characteristics can be obtained by such a configuration is as follows. That is, when the phase plate 18 having the positive uniaxial optical medium 19 is divided, the positive phase plate 18 is arranged symmetrically as viewed from the liquid crystal layer 1. Further, when the phase plate 6 having the negative uniaxial optical medium 12 is divided, in addition to the increase in symmetry, the positive and negative birefringence characteristics should be canceled in the depth direction as described in the fourth embodiment. The distance will be short. Therefore, if the negative phase plate 6 is divided, a larger viewing angle characteristic improvement effect can be obtained.
[0124]
  In the fifth embodiment and its modifications, the liquid crystal layer 1 and the phase plates 18 and 6 are arranged in this order as viewed from the light source side. However, the stacking order of the liquid crystal layer 1 and the phase plates 18 and 6 is not limited to this, and these layers are stacked in another order as long as they are sandwiched between the two polarizing plates 7 and 8. Alternatively, the backlight may be arranged on the polarizing plate 7 side and the display may be observed from the polarizing plate 8 side. Also in this case, a good viewing angle characteristic can be obtained by the same principle as described in the fifth embodiment.
[0125]
  In addition, it is desirable that the angles formed by the liquid crystal molecules and the principal axes of the optical medium in the proximity surfaces of the liquid crystal layer 1 and the phase plates 6 and 18 are within 10 degrees in all the proximity surfaces. That is, the angle formed between the alignment direction of the liquid crystal molecules 11 at the phase plate side interface 1d of the liquid crystal layer 1 and the main axis direction of the positive uniaxial optical medium 19 at the liquid crystal layer side interface 18a of the phase plate 18 in FIG. And the main axis direction of the positive uniaxial optical medium 19 at the phase plate 6 side interface 18b of the phase plate 18 and the main axis direction of the negative uniaxial optical medium 12 at the liquid crystal layer side interface 6e of the phase plate 6. It is desirable that all angles be within 10 degrees.
[0126]
  Further, in the fifth embodiment and the modification thereof, when the main axis of the negative uniaxial optical medium of the phase plates 18 and 6 has a tilt angle, between the phase plate 18 and the liquid crystal layer 1, and At each interface of the phase plates 18 and 6, the tilt angles are close to each other at the interfaces (interface 18 a and interface 1 d and interface 18 b and interface 6 e) close to each other between the phase plate 6 and the liquid crystal layer 1. It is desirable to set the direction of the main axis and the pretilt angle of the positive / negative uniaxial optical medium.
[0127]
  Furthermore, in the fifth embodiment and its modifications, the angle formed between the polarization axis of the polarizing plate 8 and the orientation direction of the liquid crystal molecules at the interface 1d of the liquid crystal layer 1 is 15 degrees to 75 degrees. If it is set in the range of 30 ° to 60 °, more desirably, the luminance-voltage characteristics are steep and liquid crystal characteristics suitable for multiplex driving can be obtained.
[0128]
  In the fifth embodiment and its modification, a polarizing plate is used when the display voltage no application state (off state) is black display (NB display) and the viewing angle dependency of black display is good. 7 may be arranged so as to be shifted from the direction orthogonal to the polarization axis direction of the polarizing plate 8 by the sum of the twist angles, but if adjusted within a range of several degrees to several tens of degrees from the orthogonal direction, Good display characteristics can be obtained. If you want white display (NW display) when no display voltage is applied (NW display) and want to improve the viewing angle characteristics of the white state, the polarization axes of the polarizing plates 7 and 8 can be considered in the same way as described above with reference to the parallel direction. That's fine.
[0129]
  The above is the description of the embodiment in which the present invention is implemented in the STN type liquid crystal display device. Next, the embodiment in which the present invention is implemented in the TN type liquid crystal display device will be described.
Sixth embodiment
  Since the configuration of the liquid crystal display device of the present embodiment is basically the same as that shown in FIG. 1, the structure will be described with reference to FIG. This liquid crystal display device includes a liquid crystal 40 made of TN liquid crystal having a twist angle of 90 degrees, and a phase plate 41 in which a negative uniaxial optical medium (not shown) is twisted by 90 degrees in the direction opposite to the liquid crystal layer 40. It has. The retardation of the liquid crystal layer 40 and the phase plate 41 is substantially equal, the alignment direction of liquid crystal molecules (not shown) at the phase plate side interface 40a of the liquid crystal layer 1, and the negative uniaxiality at the liquid crystal layer side interface 41a of the phase plate 41. It is substantially parallel to the principal axis direction of the optical medium. Although not shown, a TFT as a voltage switching element is formed on the transparent substrate 42. This TFT can be replaced by a diode element such as MIM.
[0130]
  In the liquid crystal display device of the present embodiment, the polarization axis of the polarizing plate 8 is parallel to the alignment direction of the liquid crystal molecules at the polarizing plate 8 side interface 40b of the liquid crystal layer 40 in order to perform the TN mode operation. They are arranged orthogonally. The polarizing axis of the polarizing plate 7 is arranged so as to be orthogonal to the polarizing axis of the polarizing plate 8, and the polarizing plates 8 and 7 are arranged in this way to perform NB display.
[0131]
  In this liquid crystal display device, when the retardation of the liquid crystal layer 40 is too small, the brightness at the time of applying the display voltage is not sufficient. On the other hand, when the retardation of the liquid crystal layer 40 is too large, the viewing angle compensation characteristic is deteriorated. Or the thickness (cell thickness) of the liquid crystal layer 40 is increased and the response speed is decreased. Therefore, the retardation of the liquid crystal layer 40 is preferably set to 300 nm or more, and more preferably set to around 400 to 500 nm.
[0132]
  Also in this embodiment, the principle of optical compensation is the same as that described in the first embodiment, and black display with a very good viewing angle characteristic can be performed in a state where no display voltage is applied. .
[0133]
  By the way, in general, an active matrix type liquid crystal display device in which a switching element is provided on a transparent substrate has a high contrast and good halftone characteristics, so that it is used as a display for displaying a full color image by mixing colors of RGB three color pixels. There are many cases. In this case, the fact that the black display does not change depending on the viewing angle is particularly important in terms of expanding the viewing angle range viewed from the display contrast and eliminating the change in display hue due to the viewing angle. In the conventional liquid crystal display device based on the NW display, when the display voltage to be applied is insufficient, the black display characteristic is deteriorated, so that the viewing angle characteristic is extremely deteriorated. On the other hand, in the liquid crystal display device of the present embodiment, good black display is realized in a state where the display voltage is zero, so that a sufficient voltage is applied to the liquid crystal layer 40 due to the withstand voltage of the driving IC and the restriction of the driving signal. Even when the voltage cannot be applied, the white luminance is slightly lowered, and the viewing angle dependence of contrast and hue is kept very good. This is because the viewing angle characteristics do not deteriorate significantly when the power supply voltage cannot be obtained sufficiently with portable devices, when it is desired to reduce power consumption, or when it is desired to reduce the radiation of electromagnetic waves from the mounted devices. It leads to the advantage of being good.
[0134]
  Table 1 shows the experimental results comparing the display characteristics when the liquid crystal display device of the present invention and two types of NW display liquid crystal display devices (conventional examples) are driven at drive voltages of 5 volts and 3.3 volts. is there.
[0135]
[Table 1]

  That is, consider a case where the NB liquid crystal display device of the present embodiment is driven with two types of signal voltages of 3.3 volts and 5 volts. In this case, the 3.3 volt drive reduces the white display transmittance by about 10% compared to the 5 volt drive, but the black display when viewed from the front has a very good contrast of 100 to 1 or more. Can be obtained, and a result equivalent to a 5-volt drive can be obtained. On the other hand, in NW display, when 3.3 volt drive is used, both systems cannot obtain sufficient contrast.
[0136]
  The viewing angle characteristics when the liquid crystal display device of the present embodiment is driven with a signal voltage of 3.3 volts are the same as those of the NB display liquid crystal display device provided with the phase plate which is a feature of the present invention. Compared with the case of driving with voltage, the conventional NW type liquid crystal display device having no phase plate, which is a feature of the present invention, is driven with a signal voltage of 5 volts or with a conventional type of phase plate. This is much better than when the NW display liquid crystal display device is driven by 3.3 volts.
[0137]
  Further, in the liquid crystal display device of the present embodiment, since a low-viscosity liquid crystal material can be used in the same manner as NW display 5-volt drive, the response speed is sufficiently good at 5 Omsec or less.
[0138]
  Therefore, the liquid crystal display device of the present embodiment has the feature that it can be incorporated into various devices of 3.3 volt logic including a portable computer without adding a booster circuit for the liquid crystal display device. It can contribute to downsizing and cost reduction of equipment. In addition, the power consumption of the liquid crystal display device is reduced to about half, so that the battery life of the portable device can be extended and the battery portion can be reduced in weight.
[0139]
  In the above description, the liquid crystal layer and the phase plate have the same retardation and twist angle, the alignment direction of liquid crystal molecules (not shown) at the phase plate side interface 40a of the liquid crystal layer 1, and the liquid crystal layer of the phase plate 41. Although the main interface direction of the negative uniaxial optical medium at the side interface 41a is substantially parallel, the present embodiment is not limited to this.
[0140]
  That is, the angle formed between the alignment direction of liquid crystal molecules (not shown) at the phase plate side interface 40a of the liquid crystal layer 1 and the main axis direction of the negative uniaxial optical medium at the liquid crystal layer side interface 41a of the phase plate 41. Is 10 degrees or less, more desirably 5 degrees or less, and the total twist angle between the liquid crystal layer 40 and the phase plate 41 is between −10 degrees and +10 degrees, taking into account the sign with one twist direction being positive. If so, practically sufficient display characteristics can be obtained.
[0141]
  Further, it is desirable that the twist angles of the liquid crystal layer 40 and the phase plate 41 are both in the range of 70 degrees to 110 degrees. In these cases, the display characteristics are further improved by slightly adjusting the directions of the polarization axes of the two polarizing plates 7 and 8 from the above-described directions.
[0142]
  Furthermore, in an actual liquid crystal display device, as described above, since the signal processing associated with the gamma characteristic of the liquid crystal and the characteristics of the switching element on the transparent substrate are insufficient, a slight voltage is applied to the liquid crystal layer when no display voltage is applied. (Off voltage) is applied. Due to such an off-voltage, the liquid crystal molecules tend to rise slightly even when no display voltage is applied.
[0143]
  In view of the above, in the liquid crystal display device of the present embodiment, it is desirable to set the retardation of the phase plate 41 to be equal to or slightly smaller than the retardation of the liquid crystal layer 40. According to experiments, it is found that if the retardation of the phase plate 41 is set between 95% and 100% of the retardation of the liquid crystal layer 40, good characteristics are given to most signal processing circuits and switching elements. ing.
[0144]
  If the main axis of the negative uniaxial optical medium included in the phase plate 41 is tilted instead of reducing the retardation of the phase plate 41, a better viewing angle characteristic can be obtained. Similarly to the second embodiment, if the equivalent retardation of the phase plate 41 is between 95% and 100% of the retardation of the liquid crystal layer 40, most signal processing circuits and switching elements are used. On the other hand, good characteristics can be obtained.
[0145]
  When the tilt angle of the main axis of the negative uniaxial optical medium included in the phase plate 41 has a distribution in the thickness direction of the phase plate, there is a portion where the tilt angle of the negative uniaxial optical medium is too large compared to the tilt angle of the liquid crystal molecules. This is not preferable because the viewing angle characteristic of the portion becomes too large. In this case, it is desirable to set the maximum value of the tilt angle of the negative uniaxial optical medium so as not to exceed three times the pretilt angle of the liquid crystal molecules included in the liquid crystal layer 40, and twice the pretilt angle of the liquid crystal molecules. It is further desirable to set the tilt angle of the negative uniaxial optical medium so as not to exceed.
[0146]
  Further, when the alignment state of the liquid crystal molecules in the state in which no display voltage is applied to the liquid crystal layer 40 is known by simulation or the like, the negative polarity of the phase plate 41 is the same as described in the third embodiment. If uniaxial optical media are arranged correspondingly, even better viewing angle characteristics can be obtained.
Seventh embodiment
  Although the liquid crystal display device of the present embodiment has a TN type liquid crystal layer 40, the configuration is basically the same as that shown in FIG. 13 (fourth embodiment). The description will be given with reference. In the present embodiment, the same or similar parts as those in the fourth embodiment are denoted by the same reference numerals.
[0147]
  The liquid crystal display device according to the present embodiment has a configuration in which first and second phase plates 41A and 41B divided into two pieces are provided, and these phase plates 41A and 41B are arranged on the front and back as viewed from the liquid crystal layer 40. ing. Also in this embodiment, the viewing angle characteristics are improved and the viewing angle symmetry is also improved by the same principle as described in the fourth embodiment.
[0148]
  Note that the angles formed by the liquid crystal molecules and the principal axes of the optical media on the adjacent surfaces of the liquid crystal layer 40 and the first and second phase plates 41A and 41B are within 10 degrees, more preferably within 5 degrees, with respect to all the adjacent surfaces. It is good to make it. That is, the alignment direction of liquid crystal molecules (not shown) at the first phase plate side interface 40b of the liquid crystal layer 40 and the negative uniaxial optical medium (not shown) at the liquid crystal layer side interface 41b of the first phase plate 6A. The angle formed by the main axis direction is preferably 10 degrees, more preferably within 5 degrees. Similarly, the alignment direction of liquid crystal molecules (not shown) at the second phase plate side interface 40c of the liquid crystal layer 40 and the negative uniaxial optical medium (not shown) at the liquid crystal layer side interface 41c of the second phase plate 41B. The angle formed by the main axis direction is preferably 10 degrees, more preferably within 5 degrees.
[0149]
  When the main axis of the negative uniaxial optical medium of the first and second phase plates 41A and 41B has a tilt angle, it is between the first phase plate 41A and the liquid crystal layer 40, and the second phase plate 41B. And the liquid crystal layer 40, the negative angles at the interfaces 41b and 41c of the phase plates 41A and 41B are such that the tilt angles are close to continuity at the interfaces close to each other (interface 41b and interface 40b or interface 41c and interface 40c). It is desirable to set the principal axis direction and the pretilt angle of the uniaxial optical medium.
[0150]
  In addition, the arrangement angle of the polarizing plates 7 and 8 may be as follows. That is, the main axis direction of the negative uniaxial optical medium and the polarization axis direction of the polarizing plate 8 at the polarizing plate 8 side interface 41d of the second phase plate 41B are arranged in parallel or orthogonal to each other, and the phase plate 7 of the first phase plate 41A. Basically, the main axis direction of the negative uniaxial optical medium at the side interface 41a and the polarization axis direction of the polarizing plate 7 are arranged orthogonally when the former is parallel arrangement, and arranged parallel when the former is orthogonal arrangement. And Then, the arrangement angle of the polarizing plates 7 and 8 may be adjusted within this range up to about 10 degrees in accordance with other parameters.
[0151]
  The retardation and twist angle of the first and second phase plates 41A and 41B may be as follows. That is, the sum of the retardations of the first and second phase plates 41A and 41B and the sum of the twist angles may satisfy the conditions described for the single phase plate 41 in the sixth embodiment. Even if the first and second phase plates 41A and 41B are not divided so that optical characteristics such as retardation are not necessarily equal, the effects described here are exhibited.
Eighth embodiment
  Although the liquid crystal display device of the present embodiment has a TN type liquid crystal layer 40, the configuration is basically the same as the liquid crystal display device of the fifth embodiment shown in FIG. A description will be given with reference to FIG. In the present embodiment, the same or similar parts as those in the fifth embodiment are denoted by the same reference numerals.
[0152]
  The liquid crystal display device according to the present embodiment is characterized by a phase including a uniaxial optical medium (negative uniaxial optical medium) having negative refractive index anisotropy twisted in the opposite direction to the TN liquid crystal layer 40. In addition to the plate (negative phase plate) 41, a phase plate (positive phase) provided with a uniaxial optical medium (positive uniaxial optical medium) having positive refractive index anisotropy twisted in the same direction as the liquid crystal layer Plate) 43. The configuration of the phase plate 43 is the same as that of the positive phase plate 18 described in the fifth embodiment.
[0153]
  Also in the configuration shown in the present embodiment, the positive phase plate 43 relaxes or averages the azimuth angle dependence of the liquid crystal display characteristics based on the same principle as described in the fifth embodiment, and the negative phase plate Since 41 compensates for this viewing angle dependence, the viewing angle characteristics are further improved as compared with the sixth embodiment.
[0154]
  In the fifth embodiment, the total of the twist angle of the liquid crystal layer 1 and the twist angle of the positive phase plate 18 is set to about 360 degrees. Also in this embodiment, if the total of the twist angle of the liquid crystal layer 40 and the twist angle of the positive phase plate 43 is set to about 360 degrees, the viewing angle dependency can be dispersed in each direction. Since the layer 40 has a twist angle of about 90 degrees, which is narrower than that of the STN type liquid crystal layer 1, if the total twist angle is 180 degrees or more, a practically sufficient viewing angle expansion effect can be obtained.
[0155]
  In the configuration of the present embodiment, it is desirable that the relationship between the twist angles of the liquid crystal layer 40 and the phase plates 43 and 41 be in the range of the following equation (9).
[0156]
                −10 ° ≦ ΩLC + Ωp + Ωn ≦ 10 ° (9)
      ΩLC: twist angle of liquid crystal molecules constituting the liquid crystal layer 40
       Ωp: twist angle of the positive uniaxial optical medium constituting the positive phase plate 43
       Ωn: twist angle of negative uniaxial optical medium constituting negative phase plate 41
  The twist direction is positive for left twist and negative for right twist, and the retardation of the phase plates 43 and 41 is equivalent when the uniaxial optical medium of the phase plates 43 and 41 has an inclined array. Retardation.
[0157]
  Next, a desirable relationship of retardation between the liquid crystal layer 40 and the phase plates 41 and 43 will be described. In the present embodiment, it is desirable that the positive phase plate 43 has a structure that is regarded as an extension of the liquid crystal layer 40. Therefore, the retardation Rp is as follows.
The value of Rpo obtained by the equation (10) may be considered as a basis.
[0158]
                    Rpo = RLC × Ωp / ΩLC (10)
      RLC: retardation of the liquid crystal layer 40
       Ωp: twist angle of the positive uniaxial optical medium constituting the positive phase plate 43
      ΩLC: twist angle of liquid crystal molecules constituting the liquid crystal layer 40
  As described in the sixth embodiment, in an active matrix type liquid crystal display device, liquid crystal molecules often rise slightly even when no display voltage is applied (off state). For this reason, the retardation of the positive phase plate 43 is set to be about 2 to 3% smaller than the value determined by the above equation (10), or the positive uniaxial optical medium constituting the positive phase plate 43 is tilted. As a result, better display characteristics can be obtained.
[0159]
  The tilt alignment direction of the positive uniaxial optical medium constituting the positive phase plate 43 is preferably matched with the tilt direction of the liquid crystal molecules.
[0160]
  In the configuration of the present embodiment, a part of the retardation of the negative phase plate 41 compensates for the characteristics of the positive phase plate 43, and the retardation of the remaining part of the negative phase plate 41 serves as the characteristics of the liquid crystal layer 40. Is considered to compensate. Therefore, the portion of the retardation of the negative phase plate 41 that compensates for the characteristics of the liquid crystal layer 40 is Rneff−Rpeff. For the same reason as described in the sixth embodiment, it is desirable that the relationship of the following expression (11) holds between Rneff−Rpeff and RLC.
[0161]
              0.95 ≦ (Rn−Rpeff) / RLC ≦ 1 (11)
  Also in the present embodiment, between the liquid crystal layer 40 and each of the phase plates 43 and 41, the angles formed by the liquid crystal molecules and the principal axes of the positive and negative uniaxial optical media on the mutually adjacent surfaces are all It is desirable that the angle be within 10 degrees on the proximity surface of the film, and it is more desirable that the angle be 5 degrees or less. Further, when the main axes of the positive and negative uniaxial optical media in the phase plates 43 and 41 have a tilt angle, the tilt angles of the liquid crystal layer 40, the liquid crystal molecules of the phase plates 43 and 41, and the main axes of the positive and negative uniaxial optical media. It is desirable to set the orientation direction as follows. That is, between the liquid crystal layer 40 and the phase plate 43, and between the phase plate 43 and the phase plate 41, the tilt angle and the alignment direction described above are set so that the tilt angles are close to each other on the adjacent surfaces. It is desirable to do.
[0162]
  Further, the arrangement angles of the polarizing plates 7 and 8 are set as follows. That is, in the polarizing axis of the polarizing plate 8 and the liquid crystal layer 1 adjacent thereto, the polarizing axis of the polarizing plate 8 and the principal axis direction of the liquid crystal molecules of the liquid crystal layer 1 are parallel or orthogonal to each other on the adjacent surface side. The principal axis direction is basically shifted from the arrangement perpendicular to the principal axis direction of the polarizing plate 8 by the sum of the torsion angles expressed by the equation (9). Then, based on such a basic arrangement, the arrangement angle of the polarizing plates 7 and 8 may be adjusted within a range up to about 10 degrees according to other parameters. The twist angle and retardation of the liquid crystal layer 40 are desirably set in the same range as in the sixth embodiment.
Modified example of the eighth embodiment
  In the liquid crystal display device according to the eighth embodiment, when one or both of the positive and negative phase plates 43 and 41 are divided into two phase plates and arranged one by one above and below the liquid crystal layer 40, it is even better. Display characteristics can be obtained.
[0163]
  This is considered to be due to the same reason as described in the third modification of the fifth embodiment. If the negative phase plate 41 is divided, a larger viewing angle characteristic improvement effect is obtained. It is done.
[0164]
  The angle between the liquid crystal layer 40 and the principal axes of the positive and negative uniaxial optical media on the adjacent surfaces of the phase plates 43 and 41 and the arrangement angle of the polarizing plates 7 and 8 are the same as in the eighth embodiment. What is necessary is just to set to the range described in the form.
[0165]
  Next, a method for manufacturing a phase plate having negative refractive index anisotropy used in each of the above embodiments will be described.
[0166]
  A phase plate in which an optical medium having negative refractive anisotropy is twisted is formed by forming an optical layer in which nematic liquid crystals are arranged in a polymer network and then replacing the nematic liquid crystals with discotic liquid crystals. be able to. This is to achieve a desired alignment distribution by replacing a discotic liquid crystal in which it is difficult to control the direction and imparting a twisted structure with a nematic liquid crystal that can be controlled relatively easily. As a more specific and preferable manufacturing method, there is a method shown in a process diagram in FIG.
[0167]
  First, as shown in 18 (a), the surface of an optically isotropic film substrate 101 such as cellulose triacetate is subjected to an orientation treatment by a rubbing treatment using a rubbing roll 102 or the like. At this time, an alignment film such as polyimide may be formed on the surface of the film substrate 101.
[0168]
  Next, as shown in FIG. 18B, a mixture 103 of nematic liquid crystal and a photopolymerizable polymer is applied onto the film equipment 101 by a roll coating method. In the figure, reference numeral 104 denotes a roll of a roll coater for applying the mixture 103. The mixture 103 may be dissolved in a solvent if necessary. The mixture 103 may be applied by spin coating or various printing methods.
[0169]
  In this way, a slight chiral substance (for example, S-811 manufactured by Merck & Co., Inc.) is added to the nematic liquid crystal to be applied to the film substrate 101 together with the photopolymerizable polymer to give a twisted structure. On the other hand, an ultraviolet curable acrylic resin or the like is used as the photopolymerizable polymer. In the case where the mixture 103 is dissolved in a solvent, the solvent is skipped by drying before the step described below.
[0170]
  Next, as shown in FIG. 18C, the mixture 103 is irradiated with ultraviolet light 105 to polymerize the photopolymerizable polymer to form a polymer network in the mixture 103. The arrangement of nematic liquid crystal molecules having a twisted structure is stabilized by the polymer network thus formed. The irradiation light may be visible light as long as the photopolymerizable polymer has a wavelength that forms a network.
[0171]
  Thereafter, as shown in FIG. 18 (d), a solvent 106 for dissolving a nematic liquid crystal such as methanol or ethanol is prepared, and a film substrate 101 on which a polymer network is formed is leached in the solvent 106, Thus, the nematic liquid crystal is selectively removed from the mixture 103 on the film substrate 101. Instead of soaking, the nematic liquid crystal can be selectively removed by pouring the solvent 106 onto the film substrate 101 in a shower.
[0172]
  Next, as shown in FIG. 18 (e), the discotic liquid crystal 108 is dropped by the syringe 109 onto the film substrate 101 pulled up from the solvent 106. As the discotic liquid crystal 108, for example, a triphenyllene compound or a benzene derivative in which a long-chain or plate-like functional group is arranged as a side chain of a benzene ring is used.
[0173]
  The discotic liquid crystal 108 enters and disperses into the polymer network 107 in which holes are formed by removing the nematic liquid crystal. Since the holes in the nematic liquid crystal mark formed in the polymer network 107 have anisotropy corresponding to the twisted structure of the nematic liquid crystal, the discotic liquid crystal 108 penetrating into the holes also has a twisted structure and a different shape. A direction is given. The shape of this twisted structure can be controlled by adjusting the amount of the chiral substance added to the nematic liquid crystal described above. In such a process, if the fluidity of the discotic liquid crystal 108 is increased by heating the syringe 109 or the film 101 substrate, the process time can be shortened or a substance that does not take a liquid crystal phase at room temperature can be used. be able to.
[0174]
  Further, instead of the dropping operation with the syringe 109, the film base material 101 may be immersed in the discotic liquid crystal 108, or the discotic liquid crystal 108 may be applied on the film base material 101 by a roll coating method or a printing method. Good. Further, instead of using the discotic liquid crystal 108 alone, a solution in which the discotic liquid crystal 108 is dissolved in an appropriate solvent is prepared, and the discotic liquid crystal 108 dissolved in this solvent is dropped or applied on the film substrate 101. May be. In this case, it is necessary to remove the solvent after the discotic liquid crystal 108 has penetrated the film substrate 101.
[0175]
  Next, as shown in FIG. 18F, the excess discotic liquid crystal 108 is removed. As a result, an optical layer 110 in which a discotic liquid crystal 108 having a twisted structure is dispersed in the polymer network 107 is formed, and the film base 101 having the optical layer 110 is formed of the above-described negative refractive index anisotropic. The phase plate has the property. In addition, it is desirable that the addition amount of the polymer network 107 is about 2-3% in weight ratio. As a result, the optical influence on the discotic liquid crystal 108 is reduced.
[0176]
  When the fluidity of the discotic liquid crystal 108 in the optical layer 110 thus formed is low and the physical stability of the phase plate is good, it may be used as it is as a phase plate. However, otherwise, when the fluidity of the discotic liquid crystal 108 is high, a protective film film that is optically isotropic with respect to the optical layer 110 such as a cellulose triacetate film is formed on the surface of the optical layer 110. Forming and protecting the optical layer 110 increases the optical stability and mechanical stability or strength of the phase plate. Moreover, when laminating | stacking a polarizing plate and another phase plate on the optical layer 110, these thin plates can also be substituted for the protective film film mentioned above. Further, when the fluidity of the discotic liquid crystal 108 is high, it is desirable to cut the optical layer 110 and the film substrate 101 into a predetermined size and then seal the cut surface.
[0177]
  In the manufacturing method described with reference to FIG. 18, the optical layer 110 is formed on the film substrate 101 to form a phase plate, but the surface of the phase plate, polarizing plate, or liquid crystal panel is used instead of the film substrate 101. A similar optical layer may be formed.
[0178]
  Furthermore, this manufacturing method is particularly suitable for continuously forming a phase plate on a roll-shaped film substrate 101 ′ as shown in the process conceptual diagram of FIG. In FIG. 19, the same or similar parts as those in FIG. 18 are denoted by the same reference numerals, and detailed description thereof will be omitted.
[0179]
  The film base 101 ′ is continuously supplied from the feed-side roll 121, and the optical layer 110 (not shown in FIG. 19) is formed on the surface through the same process as described in FIG.Been formedLater, it is wound around a roll 122 on the receiving side.
[0180]
  The film substrate 101 ′ on which the optical layer 110 is formed may be subjected to processing such as cutting immediately after this, or may be laminated with another film such as a phase plate or a polarizing plate in the form of a roll. Good. If continuous production is carried out in this way, the production amount is greatly expanded, and a large amount of phase plates having negative refractive index anisotropy can be supplied at low cost.
[0181]
  The phase plate having negative refractive index anisotropy manufactured in this way has a continuous twisted structure and therefore has good optical characteristics. This is the liquid crystal display according to the first to eighth embodiments. When used in the apparatus, very good display characteristics can be obtained. Moreover, if continuous production using a roll-shaped film is performed, it can contribute to the cost reduction of a liquid crystal display device further.
[0182]
  In the above-described method for manufacturing a phase plate having negative refractive index anisotropy, a discotic liquid crystal 108 ′ in which a reactive group is formed at the end may be used as the discotic liquid crystal 108. That is, the discotic liquid crystal 108 ′ described above is permeated into the polymer network 107 in which holes are formed by removing the nematic liquid crystal, as in FIG. Further, a cross-linking reaction is performed between the discotic liquid crystals 108 ′ and between the polymer network 107 and the discotic liquid crystals 108 ′. The crosslinking reaction can be caused by heating or light irradiation.
[0183]
  In this way, a highly stable optical layer 110 ′, and thus a phase plate, can be obtained by crosslinking between the discotic liquid crystals 108 ′ and between the polymer network 107 and the discotic liquid crystals 108 ′. Can do. In this case, it is not necessary to seal the side surface of the phase plate. Further, in particular, when it is not necessary to protect the upper surface (or lower surface) of the phase plate from scratches, the optical layer 110 ′ having the discotic liquid crystal 108 ′ may not be covered with a protective film.
[0184]
  Further, in the method of manufacturing a phase plate whose stability is improved by crosslinking, the following may be performed. That is, after creating the configuration shown in FIG. 18 (f), the cross-linking reaction is caused only in the discotic liquid crystal 108 ′ by selecting the polymer material and the end group of the discotic liquid crystal 108 ′ and setting the cross-linking reaction conditions.
[0185]
  Thereafter, as shown in FIG. 20A, only the polymer network 107 is removed from the optical layer 110 ′ by a solvent 111 such as toluene and xylene, which selectively dissolves only the polymer network 107 in the optical layer 110 ′. Selectively remove. As a result, the optical layer 112 made only of the discotic liquid crystal 108 ′ is formed on the film substrate 101.
[0186]
  Next, as shown in FIG. 20B, a discotic liquid crystal 114 having a reactive group formed at the end is prepared, and this discotic liquid crystal 114 is supplied onto the optical layer 112 by a dropping operation using a syringe 113 or the like. Then, the discotic liquid crystal 114 is permeated into the pores of the polymer network 107 formed in the optical layer 112. Further, the discotic liquid crystal 114 is crosslinked to form an optical layer 115 shown in FIG.
[0187]
  In the phase plate thus obtained, the polymer network 107 of the optical layer 110 shown in FIG. 18 (f) is replaced with the discotic liquid crystal 114, and the entire optical layer 115 is formed of the discotic liquid crystal. Therefore, the phase plate manufactured in this way has the following advantages. That is, in order to increase the mechanical stability of the phase plate produced by the manufacturing method described with reference to FIG. 18, the amount of the polymer network 107 in the optical layer 110 needs to be increased. However, when the polymer network 107 is increased, an optical adverse effect on the discotic liquid crystal 108 (for example, change due to light scattering due to a difference in refractive index) increases accordingly. That is, in the phase plate produced by the manufacturing method of FIG. 18, the optical characteristics and the mechanical stability are in a contradictory relationship.
[0188]
  In contrast, in the manufacturing method in which the optical network 107 is replaced with the discotic liquid crystal 114, the number of steps is increased, but the adverse optical effects of the polymer network 114 can be eliminated, and the optical characteristics and mechanical stability can be eliminated. It is possible to balance the sex.
[0189]
  In each of the phase plate manufacturing methods described above, when a polymer network is formed by irradiating the mixture 103 of nematic liquid crystal and photopolymerizable polymer to form a polymer network, an electric field (or magnetic field) is applied in the normal direction of the film. The light irradiation may be performed. Then, since the nematic liquid crystal molecules rise in the normal direction of the film substrate 101, the principal axis direction of the discotic liquid crystal 108 finally obtained is tilted from the normal direction of the film base substrate 101, or the film base It is possible to provide a tilt angle distribution in the depth direction of the material 101.
[0190]
  Thereby, the phase plate described in the second embodiment, that is, the phase plate in which the main axis direction of the negative uniaxial optical medium 15 is matched with the molecular arrangement of the liquid crystal layer 1 can be obtained.
[0191]
  In the phase plate produced by the method of manufacturing the phase plate having negative refractive index anisotropy described above, the interface between the optical layer 110 in contact with the film substrate 101 and the interface between the exposed optical layer 110 In some cases, the tilt angles may be different from each other, and such a configuration is not convenient when a symmetrical tilt structure is required from the optical characteristics of the display device.
[0192]
  Therefore, when a symmetrical tilt structure is required, a plurality of phase plates produced by the above-described method for producing a phase plate having negative refractive index anisotropy are connected to each other at a negative uniaxial optical interface. A symmetric tilt structure can be obtained by stacking so that the main axes of the medium are substantially continuous to form a single phase plate.
[0193]
  In this case, it is desirable to remove the film substrate 101 from the phase plates other than both ends in order not to increase the optical characteristics of the phase plate as a whole or increase the thickness of the phase plate. In addition, when the number of stacked layers is large, the display characteristics deteriorate due to the influence of the interface. Therefore, the number of stacked layers is preferably 3 or less.
[0194]
  Further, in the above-described method for manufacturing a phase plate, the discotic liquid crystal 108 is twisted to produce a negative phase plate having a main axis twisted. However, in these manufacturing methods, if the structure of the nematic liquid crystal to be created first is controlled to take a different arrangement from the twisted arrangement, it is possible to produce negative phase plates with other arrangements. It is. For example, if no chiral agent is added, the nematic liquid crystal can be vertically aligned, horizontally aligned, tilted or hybrid aligned depending on the alignment conditions at the interface and the application conditions of the electric field and magnetic field during photocuring. This makes it possible to produce a negative phase plate whose optical axis is vertically aligned, horizontally aligned, tilted or hybridly aligned.
[0195]
【The invention's effect】
  As described above, according to the present invention, it is possible to compensate for the viewing angle dependence of the optical propagation characteristics of the liquid crystal layer in the display voltage non-application state (off state), and to obtain a liquid crystal display device having a wide viewing angle. In particular, when applied to a TN type liquid crystal display device, display with sufficiently good viewing angle characteristics in NB display can be performed, so that drive voltage can be reduced, particularly 3.3 volt drive. .
[0196]
  Further, according to the present invention, it is possible to easily form a phase plate in which the optical axes of the negative optical medium are twisted.
[0197]
  The above is the overall effect of the present invention. Next, the effect of each claim will be described in detail.
Effect of claim 1
  The viewing angle dependence of the optical propagation characteristics of the liquid crystal layer in the state where no voltage is applied is compensated, thereby improving the viewing angle characteristics, and the viewing angle dependence of the optical propagation characteristics of the liquid crystal layer in the state where no display voltage is applied is oriented. Dispersion in the angular direction compensated for viewing angle dependence. Therefore, the viewing angle characteristics are further improved accordingly.In addition, the liquid crystal layer may have a simple matrix structure, and accordingly, a liquid crystal display device with good viewing angle characteristics can be provided at a low price, thereby reducing costs. In addition, the viewing angle dependence of the optical propagation characteristics in the liquid crystal layer in the state where no display voltage is applied is distributed almost evenly in all azimuth angles, and the difference in viewing angle characteristics due to the azimuth is reduced. Furthermore, the viewing angle characteristics were improved.
Effect of claim 2
  The viewing angle compensation effect between the liquid crystal layer and the second phase plate was increased, and the viewing angle characteristics were further improved accordingly.
Effect of claim 3
  The viewing angle compensation effect of the first phase plate and the second phase plate is enhanced, and the viewing angle characteristics are further improved accordingly.
Effect of claim 4
  The tolerance of optical compensation increased, the symmetry of optical propagation characteristics increased, and the viewing angle compensation effect between the liquid crystal layer and the second phase plate further increased. This further improved the viewing angle characteristics.
Effect of claim 5
  The consistency of the optical propagation characteristics of the liquid crystal layer and the first and second phase plates is increased, and the viewing angle characteristics are further improved accordingly.
Effect of claim 6
  Combined with the active matrix, high contrast and viewing angle characteristics have been improved. In addition, the viewing angle dependence of the liquid crystal layer in the state where no display voltage is applied is broadly divided, the difference in viewing angle characteristics depending on the azimuth angle is reduced, and the viewing angle dependence is reduced.
Effect of claim 7
  The consistency of the optical propagation characteristics between the liquid crystal layer and the first and second phase plates is increased, and the viewing angle characteristics are further improved accordingly.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a configuration of a liquid crystal display device according to first to third embodiments of the present invention.
FIG. 2 is a schematic cross-sectional view of a liquid crystal layer in a liquid crystal display device of the present invention.
FIG. 3 is a schematic cross-sectional view of a phase plate in a liquid crystal display device of the present invention.
FIG. 4 is a cross-sectional view schematically showing an arrangement relationship between liquid crystal molecules and a uniaxial optical medium having negative refractive index anisotropy in the liquid crystal display device of the present invention.
FIG. 5 is a cross-sectional view for comparison of viewing angle compensation effect between a uniaxial optical medium having negative refractive index anisotropy and a uniaxial optical medium having positive refractive index anisotropy.
FIG. 6 is a characteristic diagram showing the relationship of contrast with the retardation ratio of the phase plate and the liquid crystal layer.
FIG. 7 is a view angle characteristic diagram of the liquid crystal display device of the first embodiment.
FIG. 8 is a view angle characteristic diagram of a conventional liquid crystal display device using a phase plate of a uniaxial optical medium having positive refractive index anisotropy.
FIG. 9 is a schematic cross-sectional view of a phase plate constituting a liquid crystal display device according to a second embodiment of the present invention.
FIG. 10 is a diagram illustrating a simulation result of a molecular arrangement state of a liquid crystal layer.
FIG. 11 is a diagram showing design conditions for an arrangement state of optical axes of a phase plate.
FIG. 12 is a cross-sectional view schematically showing an arrangement relationship between liquid crystal molecules and a uniaxial optical medium having negative refractive index anisotropy in a liquid crystal display device according to a third embodiment of the present invention.
FIG. 13 is a cross-sectional view showing a configuration of a liquid crystal display device according to a fourth embodiment of the present invention.
FIG. 14 is a view angle characteristic diagram of the liquid crystal display device of the fourth embodiment.
FIG. 15 is a cross-sectional view showing a configuration of a liquid crystal display device according to a fifth embodiment of the present invention.
FIG. 16 schematically shows an arrangement relationship of liquid crystal molecules, a uniaxial optical medium having negative refractive index anisotropy, and a uniaxial optical medium having positive refractive index anisotropy in a fifth embodiment. It is sectional drawing shown.
FIG. 17 is a view angle characteristic diagram of the liquid crystal display device of the fifth embodiment.
FIG. 18 is a process diagram showing an embodiment of a method for producing a phase plate according to the present invention.
FIG. 19 is a process diagram showing a modification of the method for manufacturing a phase plate of the present invention.
FIG. 20 is a process diagram showing still another modified example of the method of manufacturing a phase plate according to the present invention.
FIG. 21 is a cross-sectional view showing a configuration of a conventional liquid crystal display device.
[Explanation of symbols]
  1,40 liquid crystal layer
  6,6A, 6B, 14,16,18,41 Phase plate having negative uniaxial optical medium
  11 Liquid crystal molecules
  12, 15, 17 Negative uniaxial optical medium
  18 Phase plate having positive uniaxial optical medium
  19 Positive uniaxial optical medium

Claims (7)

A liquid crystal layer having positive refractive index anisotropy and having a liquid crystal twist angle of 200 degrees to 300 degrees;
At least one film-like first phase plate laminated on the liquid crystal layer;
Comprising at least one film-like second phase plate laminated on the liquid crystal layer,
The first phase plate has disk-like molecules having negative refractive index anisotropy and having a principal axis twisted in a direction opposite to the twist direction of the liquid crystal molecules of the liquid crystal layer,
The second phase plate has a positive refractive index anisotropy, a principal axis is twisted in the same direction as the twist direction of the liquid crystal molecules of the liquid crystal layer, and the sum of the liquid crystal twist angle of the liquid crystal layer A liquid crystal display device characterized by having a uniaxial molecule having a range of 330 degrees to 390 degrees . 2. The liquid crystal display device according to claim 1, wherein the liquid crystal molecule alignment direction at the second phase plate side interface of the liquid crystal layer and the principal axis direction of uniaxial molecules at the liquid crystal layer side interface of the second phase plate. The liquid crystal display device is characterized in that the angle formed at 10 is 10 degrees or less. 3. The liquid crystal display device according to claim 1 , wherein the main axis direction of the disk-like molecule at the second phase plate side interface of the first phase plate and the first phase plate side interface of the second phase plate are. A liquid crystal display device characterized in that an angle formed between the uniaxial molecular main axis direction is 10 degrees or less.   4. The liquid crystal display device according to claim 1, wherein the second phase plate is disposed on both surfaces of the liquid crystal layer. 2. The liquid crystal display device according to claim 1, wherein one of a twist direction of a liquid crystal twist angle of a liquid crystal layer, a principal axis twist angle of a disk-like molecule, and a principal axis twist angle of a uniaxial molecule is positive. the considering sum, - a liquid crystal display device, characterized in that within at 10 degrees + 10 degrees. A liquid crystal layer having positive refractive index anisotropy and having a liquid crystal twist angle of 70 degrees to 110 degrees;
At least one film-like first phase plate laminated on the liquid crystal layer;
Comprising at least one film-like second phase plate laminated on the liquid crystal layer,
The first phase plate has disk-like molecules having negative refractive index anisotropy and having a principal axis twisted in a direction opposite to the twist direction of the liquid crystal molecules of the liquid crystal layer,
The second phase plate has a positive refractive index anisotropy, a principal axis is twisted in the same direction as the twist direction of the liquid crystal molecules of the liquid crystal layer, and the sum of the liquid crystal twist angle of the liquid crystal layer Having a uniaxial molecule of approximately 360 degrees . 7. The liquid crystal display device according to claim 6 , wherein one of the twist direction of the liquid crystal twist angle of the liquid crystal layer, the principal axis twist angle of the discotic molecule , and the principal axis twist angle of the uniaxial molecule is positive. A liquid crystal display device characterized in that the sum in consideration of the above is -10 degrees or more and within +10 degrees.

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