LED

　

WordNet

1. the 12th letter of the Roman alphabet (同)l

PrepTutorEJDIC

1. light-emitting diode
2. lira(イタリアの貨幣単位リラ)
3. leadの過拒・過去分詞

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2017/06/06 00:05:02」(JST)

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発光ダイオード

青, 緑, 赤の LED
種類	受動素子, 光エレクトロニクス
動作原理	エレクトロルミネセンス
発明	Oleg Losev (1927) James R. Biard (1961) ニック・ホロニアック (1962)
商品化	1962年10月
ピン配置	アノード と カソード
電気用図記号

赤色発光ダイオード

発光部の拡大図。+/-で示されるのが端子の極性。発光素子の乗っている側のリードがカソード（負極）の製品が多い。

陽極（アノード、anode）と陰極（カソード、cathode）の形状の例と回路記号

上図 発光ダイオードの回路図と、電子・正孔の分布を模式的に描いた図。
下図 発光ダイオードのバンド構造と、それによる発光過程の説明。横軸が距離または位置、縦軸が電子または正孔のポテンシャルエネルギー（エネルギー準位）を表す。

半導体ダイオードの電流-電圧特性。LEDも基本的には同様の特性を示す。図示した通り、印加電圧がVfを超えると急に電流が流れ始める非直線特性を持つ。抵抗を直列に入れて傾きを緩やかにするか、能動素子で定電流制御する必要がある。

1個（共通）の陽極と2個の陰極をもつ2色LED

発光ダイオード（はっこうダイオード、英: light emitting diode: LED）はダイオードの一種で、順方向に電圧を加えた際に発光する半導体素子である。

1962年、ニック・ホロニアックにより発明された。発明当時は赤色のみだった。1972年にジョージ・クラフォードによって黄緑色LEDが発明された。1990年代初め、赤崎勇、天野浩、中村修二らによって窒化ガリウムによって青色LEDの半導体が発明された。

発光原理はエレクトロルミネセンス (EL) 効果を利用している。また、有機エレクトロルミネッセンス（OLEDs、有機EL）も分類上、LEDに含まれる。

原理

発光ダイオードは、半導体を用いたpn接合と呼ばれる構造で作られている。発光はこの中で電子の持つエネルギーを直接、光エネルギーに変換することで行われ、巨視的には熱や運動の介在を必要としない。電極から半導体に注入された電子と正孔は異なったエネルギー帯（伝導帯と価電子帯）を流れ、pn接合部付近にて禁制帯を越えて再結合する。再結合時に、バンドギャップ（禁制帯幅）にほぼ相当するエネルギーが光として放出される。放出される光の波長は材料のバンドギャップによって決められ、これにより赤外線領域から可視光線領域、紫外線領域まで様々な発光を得られるが、基本的に単一色で自由度は低い。ただし、青色、赤色、緑色（光の三原色）の発光ダイオードを用いることであらゆる色（フルカラー）を表現可能である。また、青色または紫外線を発する発光ダイオードの表面に蛍光塗料を塗布することで、白色や電球色などといった様々な中間色の発光ダイオードも製造されている。

特性

電気的特性

他の一般的なダイオードと同様に極性を持っており、カソード（陰極）に対しアノード（陽極）に正電圧を加えて使用する。電圧が低い間は電圧を上げても電流が増えず、発光もしない。ある電圧を超えると電圧上昇に対する電流の増え方が急になり、電流量に応じて光を発するようになる。この電圧を「順方向降下電圧 (V_F)」というが、一般的なシリコンダイオードと比較すると、発光ダイオードは順方向降下電圧が高い。発光色によって違うが、赤外では1.4V程度。赤色・橙色・黄色・緑色では2.1V程度。白色・青色では3.5V程度。紫外線LEDは最もV_Fが高く、4.5から6Vが必要である。

発光時の消費電流は表示灯用途では数mAから50mA程度だが、照明用途のものでは消費電力が数十Wに及ぶ大電力の発光ダイオードも市販されており、最大駆動電流が10Aに迫る製品も存在する。

逆方向に電圧を掛けた場合の耐電圧は、通常のシリコンダイオードより遙かに低く、通常はマイナス5V程度である。これを超えると破壊されるため、整流用途には使用できない。

光の特性

• 波長の偏り

蛍光灯や白熱灯など他の多くの光源と異なり、特定の波長に偏った光となっている。そのため、対応する波長に対する光化学反応が促進されたり、逆に明るさの割に必要な波長の光がないため十分な効果が得られないことがある。

有効活用としては、光源の種類によっては不要な紫外線や赤外線を含まない光が簡単に得られるため、紫外線に敏感な文化財や芸術作品、熱照射を嫌う物の照明に用いられる。特定の波長の光を好む植物の育成促進の効果もある。

逆に、明るくても特定の波長の光がないため、照明として使用した場合、動植物の育成を阻害することもある。例えば、ビタミンDの生成に必要な波長を含んでいないために欠乏症になったり、光合成に必要な波長の光を含んでいないために生育が阻害されたりする。特に、3色LED方式で白色発光を作り出している方式では、波長分布による弊害が顕著となる。

なお、蛍光体を利用して波長分布を拡散している白色タイプもあるが、その場合でも光源のもともとの波長である青色のところにひときわ強い波長の光が分布している。

• 明滅の切り替え速度

入力電流変化に対する光出力の応答が早く通信などにも利用されるほか、照明に用いた場合は点灯と同時に最大光量が得られる。

物理的特性

• 構造が簡単なため大量生産が可能。
• 価格は赤色LEDで1個5円 - 10円程度と安価。
• 電球と違いフィラメントを使わないため軽量で衝撃に強く長寿命であり、故障の発生する頻度も低い。

駆動方式

基本的に光量が電流に比例することから、定電流回路や平均電流を一定になるように制御した高周波回路で駆動する。 交流電源はダイオードブリッジなどで整流して利用される。

電流制限抵抗

定電圧電源に接続して使用する場合は、抵抗器を直列に接続する事で電流をほぼ一定にできる。

電源電圧を E として電流 I を流すには、適切な抵抗値はおよそ (E-V_F) /I となるが、LEDの順方向降下電圧 (V_F) には個体差があり、抵抗にかかる電圧が変わるため、実際に製造された製品に流れる電流は設計時に想定した値に比べて多少のバラツキが生じる。

抵抗も電力を消費するため電力効率は良くないが、定電圧電源を用意できる場合には最も単純かつ低コストな回路となる。そのため、発光効率を特に追及しない表示灯用途には多用される。

定電流駆動

定電流ダイオード (CRD) を直列に接続する等、能動素子で定電流回路を構成する事により自動車やバイクのバッテリー等、電源電圧がある程度変動する環境下でも対応できる。

電源には、LEDの順方向電圧降下に加え、定電流回路の動作に必要な電圧が必要となる。CRDは動作に5から10V程度の電圧を必要とするが、1V程度の電圧でCRDと同等の動作ができるICも利用されている。

回路は単純だが、電流制限抵抗と同様、過大な電源電圧を電力を消費して吸収するため、電源電圧によっては電力効率が悪くなる。

高周波駆動

人間の視覚が認識できない短い時間周期の点滅を繰り返し、見かけ上一定の明るさを得る。明るさは点灯時間のデューティ比を変えるパルス幅変調により容易に調節できる。

駆動回路には電力効率は良いが出力に電流・電圧に変動（リップル）があるスイッチング電源や昇圧回路を用いることが可能である。また、出力電流の平均を一定に保つことで、乾電池のように電源電圧が低かったり、変動幅が大きかったり、という場合にも一定の明るさを維持可能である。

駆動回路で消費される電力が他の駆動方式に比べ少なく、入力電力の大半がLEDで消費されるため、電力効率は比較的良い。しかし、電流断続時の急激な電流変化により生じるノイズ放射が機器内外へ電磁妨害を及ぼすほか、回路規模増大に伴ってコストと実装体積が増加する。

使用に必要な知識

• 発する光の強さは電流の量におおよそ比例する。しかし特に大電流域では効率が低下する。
• 熱に弱く、80℃以上で素子の劣化が始まるため寿命が縮む。
• 発熱が少ないとはいえ、高出力品では相応に発熱する。熱に弱いので、放熱の必要性は白熱球や蛍光灯よりむしろ高い。ヒートシンクなどで適切に放熱しないと効率の低下や寿命の短縮で発光ダイオードの利点が失われる他、発煙・発火などの事故に繋がる事がある。
• 連続最大電流、瞬間最大電流を超えないこと。定格電流より大きい電流を流すと高光束が得られるが、寿命が極端に短くなる。LEDを使用した市販品では、寿命を犠牲にして高輝度を得ている物や価格を抑えるために電流を制限する回路を省いている物もある。
• 極性があることから、アノードとカソードを間違えて印加した場合発光しない。また逆方向に対する耐電圧が低く、破壊されやすい。
• 並列接続する際は注意が必要。順方向降下電圧 (V_F) には個体差があり、並列に繋ぐと最も順方向降下電圧（簡単に言えば、電流が流れ始める電圧）の低い素子のみに電流が集中する。電流の集中でさらに発熱し電気抵抗とV_Fの値が減少し、さらに電流の集中が促進されるという悪循環が起こる。発光量が不均一になるだけでなく、電流が最大定格を超えれば過熱による寿命短縮や焼損の危険もある。素子の破壊がオープンモードだった場合は、次にV_Fの低い素子に更に大量の電流が集中し、連鎖的に破壊が進行する。複数のLEDを同時に点灯する場合は、可能な限り直列に繋いだ上で抵抗や能動素子で定電流制御した回路を1単位とし、この単位回路を並列に電源に繋ぐ。ただし、複数の素子が内部で並列接続されている製品もある。
• GaN系などの発光ダイオードは静電気やサージ電流に弱い。
• レンズ付きの発光ダイオードの場合、素子の光軸と実際に放出される光の方向は、製造過程でのばらつきのため通常一致せずわずかにずれている。
• 特定の周波数に偏った光であるため、見た目の明るさ以上に強い光であり、該当する周波数の光による光化学反応が促進される。例えば、他の発光器具にも言えることではあるが直視すると、目に悪影響を与える事がある。特に紫外線や高出力のものはその傾向が強い。

材料

放出された光の波長（色）は、pn接合を形成する素材のバンドギャップの大きさが関係する。発光ダイオードでは近赤外線や可視光、紫外線に至る波長に対応したバンドギャップを持つ半導体材料が用いられる。一般に発光ダイオードには発光再結合確率の高い直接遷移型の半導体が適する一方、一般的な半導体材料であるケイ素（シリコン）やゲルマニウムなど間接遷移型半導体では、電子と正孔が再結合するときに光は放出されにくい。しかし、黄色や黄緑色に長く使われてきたGaAsP系やGaP系などドープした不純物の準位を介して強い発光を示す材料もあり、広く用いられている。

以下の素材を使用することにより、さまざまな色の発光ダイオードを作り出すことができる。

• アルミニウムガリウムヒ素 (AlGaAs) - 赤外線・赤
• ガリウムヒ素リン (GaAsP) - 赤・橙・黄
• インジウム窒化ガリウム (InGaN) /窒化ガリウム (GaN) /アルミニウム窒化ガリウム (AlGaN) - （橙・黄・）緑・青・紫・紫外線
• リン化ガリウム (GaP) - 赤・黄・緑
• セレン化亜鉛 (ZnSe) - 緑・青
• アルミニウムインジウムガリウムリン (AlGaInP) - 橙・黄橙・黄・緑
• ダイヤモンド (C) - 紫外線
• 酸化亜鉛 (ZnO) - 青・紫・近紫外線（開発中）
• ペロブスカイト半導体 - 赤・黄・緑

以下は基板として利用されている。

• 炭化珪素基板 (SiC) - 青
• サファイア（コランダム）基板 (Al₂O₃) - 青
• ケイ素基板 (Si) - 青（研究段階）

青色発光ダイオード

青色発光ダイオード

青色発光ダイオードは主に窒化ガリウム (GaN) を材料とする、青色の光を発する発光ダイオードである。青色LEDとも書かれる。日本の化学会社、日亜化学工業株式会社が大きなシェアを占めている。他の有力メーカーとしては、豊田合成、星和電機などがある。GaN系化合物を用いた発光ダイオードの開発とそれに続く青色半導体レーザーの実現により、紫外から純緑色の可視光短波長領域の半導体発光素子が広く実用化されるに至った。

歴史

発光ダイオードは低電力で駆動することができる光源のため、ディスプレイへの応用が期待されていた。RGBによるフルカラー表示のためには光の三原色（赤・緑・青）の発光素子が揃う必要があるが、このうち1980年代中頃までに実用化されていたのは純赤色のみであった。

当時も「青色ダイオード」の名で販売されているものはあったが、色味が紫がかっており、純青としての実用的な高い輝度を出す製品は皆無だった。また黄緑色は赤色と共に早くから実用化されていたが、純緑色の実現には結果的に青色と同じくGaN系半導体材料が必要とされ、純緑色LEDの実用化は青色LEDの登場以降である。これらのことから、発光ダイオードによるフルカラーディスプレイの実現は困難だった。

純青色発光の実現のためセレン化亜鉛 (ZnSe) 系化合物や炭化ケイ素 (SiC) を用いての研究が古くから行われ、ZnSe系による青緑 - 緑色発光ダイオードの開発に至った他、SiCの青色発光ダイオードは弱い発光強度ながら市販もされた。しかしその後、GaN系化合物による青色発光ダイオードが急速に普及したため、現在ではこれらの材料系の技術は白色発光素子や基板などの用途に転用されている。

窒化ガリウムを用いた高輝度の青色LED開発に関して、1986年、赤崎勇、天野浩らが高品質、高純度のGan結晶の結晶生成に成功。天野浩は不可能とされていた「PN接合」が可能だと初めて証明した。1993年に中村修二が、世界に先駆け高輝度青色LEDを発明、実用化した(文部科学省平成28年版科学技術白書のp20とp28に記載されている)。

2001年8月、中村修二が職務上で1993年11月に発明した（職務発明）「404特許」を巡って元勤務先の日亜化学工業を提訴し、同特許の原告への帰属権確認ないし譲渡対価を巡って係争した（青色LED訴訟）。この訴訟は企業と職務発明者との関係について社会の関心を広く喚起し、裁判所は一審では発明の対価を約604億円と評価し200億円の支払いを命じたが、東京高裁は和解へと誘導し1審判決が認定した発明の対価約604億円の1/100 相当の6億円を「対価」として提示。日亜は、（いずれにせよ対価の支払いが遅れていたので）遅延損害金を含む約8億4千万円を支払うことで和解が成立した。しかし中村修二はなお納得できず、「高裁は山ほど提出した書面をまるで読まず、最初から和解金額を決めていた。高裁の和解案の決め方は正義とは言えない」と指摘するために、滞在していたアメリカより日本に訪れるという出来事もあった。

2004年12月、東北大学金属材料研究所の川崎雅司（薄膜電子材料化学）らの研究チームはより安価な酸化亜鉛を用いた青色発光ダイオードの開発に成功した。青色LEDの再発明ともいわれている。この成果は同年12月19日付の英科学誌ネイチャーマテリアルズ（電子版）にて発表している。高コストの窒化ガリウムに取って代わる可能性もある。

赤崎、天野、中村の三名は青色発光ダイオードに関する業績が評価され、2014年のノーベル物理学賞を受賞した。

白色発光ダイオード

白色発光ダイオード（点灯時）

白色LEDとも書かれる。白色光とは、一般には可視光線の全スペクトル域に渡り強度が連続している光(連続スペクトルの光)を指す用語である。発光ダイオードで得られる発光は、レーザーほどではないものの狭い波長範囲のみに限られるため、この意味での白色光を生成することはできない。しかし、白色のような多色光に対しては、スペクトルが異なっていても同一の色と人間の眼に認知させるようにスペクトルを設計することが可能である。典型的には、テレビのように光の三原色を混合したり、補色関係にある2色を混合して、適切な強度比に設計すれば白色に認知される光が生成できる。白色発光ダイオードではこの原理が利用され、具体的な手法がいくつか考案されている。この結果、低電圧でのDC駆動などダイオードの持つ電気的な扱いやすさのみならず、光源としても高効率（低消費電力）であり、しかも寿命も既存の光源以上に長いことから、LED照明として白色発光ダイオードが利用されるなど、気体を使わない固体光源として普及が進んでいる。

蛍光体方式

青またはそれよりも波長の短い光を放つ発光ダイオードのチップに、その発光ダイオードの光により励起されて長波長の光を放つ蛍光体（フォトルミネセンス）を組み合わせた方式。発光ダイオードのチップは蛍光体で覆われており、点灯させると、発光ダイオードチップからの光の一部または全部が蛍光体に吸収され、蛍光はそれよりも長波長の光を放つ。発光ダイオードのチップが青発光であれば、チップからの青色の光に蛍光体の光が混合されてともに出力される。蛍光波長や蛍光体の厚さなどを調整すれば白色光を得ることができる。この蛍光体には、例えばYAG系のものが用いられる。この方式には、単一のチップとパッケージだけで白色発光が実現可能だという利点がある。

白色に認識される光を放つような白色発光ダイオードの実現には、青色発光ダイオードの存在が不可欠であった。蛍光体による発光では、蛍光体が受けた光より短い波長の光は得られないため、赤や緑のLEDでは短波長の成分が不足し白色に認識されないからである。そして蛍光体方式の開発により、固体光源である白色発光ダイオードが本格的に普及することとなった。

擬似白色発光ダイオード

現在の白色発光ダイオードの主流であり、一般に青黄色系擬似白色発光ダイオードと呼ばれている。視感度の高い波長である黄色に蛍光する蛍光体と青色発光ダイオードとを組み合わせることによって、視覚上で大変に明るい白色発光ダイオードを実現している。青色発光ダイオードの製造を行っている日亜化学は元々蛍光体の製造メーカーであるためこの方式を得意としている。豊田合成も同方式を用いている。この方式により作成された白色発光ダイオードが、世界初の白色発光ダイオードとされている。擬似白色発光ダイオードの実現は、世界的にインパクトを与えた青色発光ダイオードの発表の後だったため報道は控えめだったが、業界内では大きなニュースとなった。

擬似白色発光ダイオードは非常に高いランプ効率 (lm/W) 値が得られることが特徴である。その理由には視感度が関連しており、視感度の高い波長にスペクトルを集中させた蛍光体の黄色と発光ダイオードの青色とを組み合わせることによって実現されている。一般に、人間の網膜にて光の強度や色を識別する細胞組織である錐体は黄緑色の波長（約555nm付近）に高い分光感度を持つ（視感度が高い）。このため、この黄緑色の波長のスペクトルに蛍光体の発光を集中させるとエネルギーの割に人は明るく感じ、視覚上大変に明るい白色発光ダイオードが実現できる。100lm/Wを超えるような白色発光ダイオードでは、ランプ効率が高い擬似白色発光ダイオードを実現するために、全光束に対するエネルギー効率が高くなるように視感度を考慮した最適化がなされている。なお、物理的なエネルギー効率は、物理エネルギー量を示す放射束を投入電力（ワット）で除算して計算されるため、光として取り出すことのできる光（光子数）を増すことにより高めることができるが、それのみでは視感度に対して効率の高くない波長域の光が多い場合もある。ランプ効率を高めるには、物理的に効率が良く、かつ、視感度に適したスペクトルが得られる必要がある。

その引き替えに、特にランプ効率を優先した設計の擬似白色発光ダイオードでは演色性が低下しやすい。一般には擬似白色発光ダイオードの平均演色評価数 (Ra) は76程度となり、一般型蛍光灯 (Ra67) と三波長型蛍光灯（同85）の中間に当たる。ただし現行の演色性の評価法は白熱灯や蛍光灯を前提としたもののため、発光ダイオードのように急峻なスペクトルを持つ光源の場合に、演色性が見た目の印象より低く評価される傾向がある。このため、前述のような特性をもつ光源について平均演色評価数がもっと高くなるように評価法を見直す議論もある。

高演色白色発光ダイオード

青色発光ダイオードと黄色蛍光体を組合わせた構成での白色光は、緑や赤のスペクトル成分が少ないため演色性が低い。赤色や深紅色の発色が悪いという性質を改善するために黄色以外の蛍光体を混ぜて演色性を改善しようとすると、ランプ効率 (lm/W) が低くなる。これは、白色発光ダイオード開発初期には青色で励起して緑や赤を発する適切な蛍光体が無く蛍光灯用の紫外線で励起される蛍光体が主体だったことと、赤色系の蛍光体を多く配合して赤色領域で多くの光エネルギーを発生させてもこの領域の人間の目の視感度が低いことからランプ効率上の評価が低くなってしまうという理由による（上述）。また、透過して出力される青色光の割合を正確に揃えることが難しく、製造時の色温度の個体差が大きい欠点もある。

これらの点について、近年は、蛍光体と発光波長の点で進展が見られる。蛍光体については、独立行政法人物質・材料研究機構がβサイアロン蛍光体の開発に成功し、これを用いることで大幅なランプ効率の向上が得られるとともに赤色や深紅色の発色の問題も解決されつつある。発光波長の点では、紫 - 紫外線を発光する発光ダイオードが開発されている（ただし、紫色発光ダイオードは紫外領域に近いため暗く見える比視感度の問題がある）。これにより、蛍光灯同様に紫光または紫外光の励起により多色を発光させ、演色性を向上させた白色発光ダイオードも登場している。

3色LED方式による白色発光

その他の白色発光の実現方法として、光の三原色である赤色・緑色・青色の発光ダイオードのチップを用いて1つの発光源として白色を得る方法もある。この方式は各LEDの光量を調節することで任意の色彩を得られるため、大型映像表示装置やカラー電光掲示板の発光素子として使用されている。ただし、照明用には適さないとされる。照明として用いることを考えた場合、蛍光体方式はある程度幅のあるスペクトルなのに対して3色LED方式は赤・緑・青の鋭い三つのピークがあるのみで黄およびシアンのスペクトルが大きく欠落している。3色LED方式の白色発光は光自体は白く見えても自然光（太陽光）の白色光とはほど遠いため、それで照らされた物の色合いは太陽光の場合と異なってくる。照らされた物の色合いが違って見える理由を説明する。

「演色性」も参照

可視光線のうち、

1. 赤色と緑色の光を反射し他を吸収する物体
2. 黄色の光のみを反射し他を吸収する物体

があったとする。太陽や白熱電球の光はあらゆる波長の可視光線を含むのでその下では、1は赤色と緑色の光が反射され網膜の赤錐体と緑錐体を刺激して黄色に見える。2は黄色の光が反射され、その光が網膜の赤錐体と緑錐体の両方を刺激して黄色に見える。つまり両者とも黄色に見える。ところが光の三原色の混合で照らした場合、1は赤と緑の光が反射され黄色に見えるが2は赤・緑・青いずれも物体に吸収されてしまい、理論上は黒く見えることになる。実際には完全に黄色の光のみを反射して他の光を一切反射しないという物体はないので黄色いはずのものが黒く見えるほどの極端なことにはならないが、多少色合いが異なって見える。蛍光灯ではこの問題を解決するために5色発光や7色発光のものがあるが、それでも演色性は白熱灯に一歩譲る。

この方式は3つのチップが必要で、見る角度に依存しない均一な発光色を得ることは難しい。さらにそれぞれのチップの要求する電圧が異なるので点灯回路も3系統必要である。しかし蛍光体が発光ダイオードのチップからの発熱で劣化する問題を回避できるメリットがある。また液晶バックライトなど表示用に用いる場合は赤・緑・青の3つの成分しか持たないことが逆に利点になり、色純度の高い鮮やかな表示色を得ることができる。

製造

発光ダイオードの基本構造はpn接合であるが、実際には発光効率を上げるためにダブルヘテロ接合構造や量子井戸接合構造などが用いられ、技術的には半導体レーザとの共通点が非常に多い。製造法としては、基板の上に化学気相成長法によって、薄膜を積み重ねていく方式などが用いられる。また、ペロブスカイト半導体ではインクジェット等の印刷技術で製造することができる。

製品の外観

7セグメント2連表示素子

5×7マトリックス表示素子

最も単純な構造は、発光部を内包する透明樹脂部分と2本の端子からなる。多色のLEDを内蔵したものは、3本以上の端子を持つ。

• 砲弾型
• チップ型
• 多セグメント形
  • 7セグメント形
  • 14セグメント形
  • マトリックス形
    • 反射型

ガリウムの資源問題

インジウムと比較してガリウムの資源は逼迫していない。しかしその産地が主に中国、カザフスタン、ウクライナに偏在し、これら各国に特有の政治的カントリーリスクの観点から、半導体材料をガリウムに依存し過ぎることに懸念が広がっている。このため酸化亜鉛やシリコン、炭化ケイ素といった材料による実用的な青色発光ダイオードの実現が急務となっている。

応用

3色LED方式（種別部分はフルカラーLED方式）を用いた駅の発車標
東京メトロ有楽町線市ケ谷駅

日本初の超高輝度LED前照灯
JR東海313系電車

動作インジケータにLEDを用いた機器

LEDを利用した信号機。太陽光などの影響を受けにくい

フルカラーLED方式を用いたJRN700系電車のLED式側面行先表示装置

白熱電球の代替として開発されたLED電球

低消費電力、長寿命、小型であるため数多くの電子機器に利用されている。特に、携帯電話のボタン照明などその特性をフルに活かして採用されているといえる。また、1つの素子で複数の色を出せるような構造のものもある。機器の動作モードによって色を変えることができるなど、機器の小型化に貢献している。

当初は輝度が小さかったため電子機器の動作表示灯などの屋内用途に限られていたが、赤色や黄緑色の高輝度タイプのものが実用化されてからは屋外でも電球式に変わり電光掲示板に採用され、さらには駅の発車標などにも使用されるようになった。

高輝度の青色や緑色、それを応用した白色の発光ダイオードが出回るようになってからは競技場のビジョンなどのフルカラーの大型ディスプレイ、電球の代わりとして懐中電灯や信号機、自動車のウィンカーやブレーキランプ、各種の照明にも利用されている。特にブレーキランプに使用した場合、電球よりブレーキペダルを踏んでから点灯するまでのタイムラグが短いため安全性が向上する。2006年には日本初となる超高輝度LEDを用いた前照灯が、JR東海313系電車で採用された。2012年5月開業の東京スカイツリーでは、夜のライトアップ照明を全てLEDで行っている。

なお、発光ダイオード自体の寿命は長いが使用目的によっては樹脂の劣化による光束低下の進行が早くなることもあり、LED交換が必要となる程度まで光束が落ちた場合に基板の交換も含む大規模なメンテナンスが必要とされるのが今後の課題となる。鉄道車両では、駅での行き先表示としての役目を果たせば良いという考えから、走行中には側面表示が一定の速度に達すると消灯するなど、きめ細かい制御で表示装置の長寿命化を図っているものも存在する。なお、編成前後の前面表示は表示のままであることが多い。ちなみに側面表示は、ドットマトリックスの制御方法から、高速移動中は表示し続けていたとしても表示文字の視認が難しい。

色覚異常によって発光ダイオードの色の見分けが困難となる場合がある。例えば1型2型の色弱の人には赤・橙・黄色・黄緑・緑のLEDは同じ色に見えてしまう。交通信号機では緑を青緑色とすることで色覚異常でも判別できるようにしているが、交通信号機以外でも色覚障害者向けの対策が必要とされる。

光通信

現代の高速通信とコンピュータを支えているのは、LEDである。サーバ内通信から家庭への通信までLEDを使った光ケーブルで行われている。また国内拠点間や海外とつなぐバックボーン（基幹）回線もほとんど光ファイバー（LED使用）によるケーブルが使われている。周波数の高い青色発光ダイオードを使うことにより、簡単に通信容量を約2倍にすることができる。

信号機

発光ダイオードの製造コストが下がり始めた2000年代以降、鉄道用および道路交通用信号機での利用も拡大している。省エネルギーで耐久性が高く、また従来白熱電球にカラーレンズを組み合わせて色を表現していた従来のものと違って、反射を最小限に抑えるクリアレンズを採用しているため太陽の反射光であたかも点灯しているかのような錯覚を見手に感じさせる疑似点灯現象の防止がなされ太陽光などの影響を受けにくいとされている。しかしながら反面、従来の白熱電球式の信号機と違い、交流電源もしくは直流でも半波整流で駆動した場合、発光原理が白熱電球と違い熱慣性がないため電源周波数に合わせて点滅してしまう。そのためタクシーなどに交通事故の証拠撮影用として搭載されているドライブレコーダーの録画周期とLEDの消灯している周期が同期してしまうと信号表示の状態が写らず、全部消灯しているように写るなどの問題が発生している。これを防ぐために国内向けの製品ではドライブレコーダーの周波数を、信号機の電源の60もしくは50Hzとずらす必要がある。また、色によっては色覚異常（色弱・色盲）の人達には見えにくい事があるため、様々な対策・研究が行われている。

積雪のある地方では、LED信号機の点灯面に雪が付着して信号が見えなくなる問題が発生している。従来は白熱電球の発熱によって融けていた着雪が、発熱の少ないLEDでは融けずに溜まってしまうためである。着雪の対策として、点灯面が凹凸の無い平面で下向きに傾けてある「フラット型」や、点灯面にアクリル樹脂製フードをかぶせた「フード型」などの着雪防止型LED信号機が開発されているものの、これといった決定打が無いのが現状である。

電光掲示板・大型映像装置

交通関連

駅の発車案内表示板や空港の発車案内板などには従来の反転フラップ式や字幕式に代わり、鉄道車両やバスの行先表示などには従来の幕式に代わり普及が進んだ。現在でもLED方向幕と呼ばれることがある。

最初に登場したLED表示機は赤色・黄緑色・橙色の3色（橙色は赤色と黄緑色LEDによる）表示方式だった。赤色LEDと黄緑色LEDにより3色目の橙色が表現されているもので、俗に「3色LED方式」とも呼ばれる。ただし、実際は2色のLEDを用いているため、工業製品などでは「2色LED」(2C-LED) とも呼称される。また、白色LEDでの赤色、青色、緑色の3色のLEDを用いた「3色LED方式」とは異なる。

その後、白色LEDを搭載したものや、単色で赤・青・緑、二色混色で黄・シアン・マゼンタ、三色混色での白の計7色を表示するマルチカラーLEDとされるもの、さらに高輝度の赤色・青色・緑色LEDによりあらゆる色を表示可能にしたフルカラーLEDのものも登場した。フルカラーLEDは、近年主流となりつつある。路線バスは鉄道ほど表示種別もなく、多くの発色を必要としないため、「3色LED」を使用しつつ交通の妨げとなりにくい橙をメインに使用する方式であったが、近年ではフルカラーLEDを採用する例も出てきている。

大型ビジョン

従来、大型ビジョンの発光素子にはCRTやVFDの光の三原色素子が利用されていたが、青色LEDの進歩によりこれらに変わってLEDが使用されるようになった。他方式に比べコストや輝度が優れており普及が進んでいる。

看板など

店頭看板などでも、従来のFL蛍光管等に代わりLEDモジュールなどのLED製品の普及が進んでいる。看板・サインのサイズの大小化や軽量化とともに故障が少なくコストに優れている。

ディスプレイのバックライト

冷陰極管が発する白色光をカラーフィルタで透過して得られる色（赤・緑・青）に比べ、RGB3色発光ダイオードが放つ光は色純度が高い。そのため、液晶ディスプレイのバックライトの光源を冷陰極管から発光ダイオードに置き換えることによって色の再現範囲を大きく広げることができる。ただし最近ではコストが安くて効率の高い擬似白色LEDが用いられることが多く、この場合は色の再現範囲は冷陰極管と変わらず、広色域タイプの冷陰極管と比べると劣る。また、LEDは点光源のため広い面積を照射しようとするとムラを生じやすく、バックライト用としては携帯機器用の小型ディスプレイに用いられることが主だったが、次第に12インチサイズ前後のノート型パソコンまで採用されるところまで来ている。

大型ディスプレイ用のLEDバックライトとしては、2004年11月にソニーより液晶テレビ「QUALIA」で実用化された。より一般的に普及が進んだのは2008年からで、各メーカーが上位機種を中心に採用するようになった。LEDテレビとは一般的に、LEDバックライトを搭載した液晶テレビのことである。2011年現在は、低価格化が進み、下位機種でも採用されることがある。エリア駆動対応機種では、映像が暗い部分のみLEDバックライトを消灯するエリア駆動により、液晶ディスプレイの弱点であるコントラストを大幅に拡大できるメリットがある。また超薄型と呼ばれる厚さを抑えた液晶テレビや、ノートパソコンの薄型化でもLEDバックライトが重要な要素となっている。また、LEDバックライトを搭載したエッジ型のディスプレイは、LEDの特性上、CCFL（蛍光管）テレビに比べて消費電力が少ない。

なお、上述の「LEDテレビ」やLEDバックライトを搭載した液晶ディスプレイ全般を指す場合に使われる「LEDディスプレイ」という呼称は、正確には誤用である。液晶テレビのバックライトは発光するための物であり、映像を表示するものではないためである。発光素子にLEDを採用した「LEDディスプレイ」については下記を参照。

LEDディスプレイ

有機発光ダイオードを採用したディスプレイについては「有機エレクトロルミネッセンス」を参照

発光素子にLEDを採用したディスプレイ。前述の大型ビジョンや街頭広告などではよく見かける。一般家庭用途などのディスプレイには、現状ではあまり開発が進んでいない。

沖データは2009年11月26日に、1.1インチQVGAの高輝度LEDディスプレイの開発に世界で初めて成功したと発表した。

また、ソニーが、「Crystal LED Display」を開発中で、2012年のCESで55型フルHDディスプレイの試作機を参考出展している。

各種照明用

省エネ、高輝度で長寿命を実現できる白色LEDの開発に伴い、発熱を伴うエネルギー消費の大きい電球に代わり新しい屋内・屋外照明材料として期待されている（LED照明）。デザインや光色なども調節できるため、より自由度の高い照明が可能になる。現在は既存の照明に置き換わる性能をもった製品が発売されており、懐中電灯、乗用車用ランプ、電球型照明、スポットライト、常夜灯、サイド照明、街路灯、道路照明灯などLEDを使用した製品が次々登場している。 日本エネルギー経済研究所が2011年に発表したリポートによると、日本全体の白熱灯や蛍光灯などをすべてLED 照明に置き換えた場合の省電力ポテンシャルとして、1時間あたり922億キロワットを節約できると試算している。これは日本の総電力消費量の約9%に相当し、原子力発電所13基分という。

E26型、E17型を中心とした白熱電球のソケットに装着可能な「LED電球」は企業間競争などにより大幅に価格が下落した。製品寿命や消費電力を考慮すれば「LED電球」の方が、白熱電球や電球形蛍光灯より低コストであると謳われているが、発売されてからまだ日が浅い商品であり、公称寿命として、各メーカーが謳う40000時間に達した例がほとんど無く、頻繁な点灯・消灯の繰り返しや連続点灯が、寿命に関わる劣化にどう影響を与えるかは未だ検証可能な個体が少なく、未知数である。

明るさや照射範囲などは「LED電球」の型番によって違いがある。より電球に近づけたと謳うものや、広配光を謳うもの、下方向のみのものなど多種多様である。中でも明るさについては、実際の明るさよりも明るいと不適切な表示（優良誤認）を行ったとして、メーカー12社に対して、2012年6月、消費者庁が景品表示法に基づく措置命令を行った。これにより、「LED電球」の明るさ基準を作る動きが生まれ、業界団体である一般社団法人日本電球工業会により、電球と置き換えた場合、電球の何ワット相当に該当するかを、全光束（ルーメン）が明るさ表示の基準として統一され出された。これにより、加盟会社の電球製品はそれぞれ電球何ワット相当と表示できる基準ルーメンと実際のルーメンに合わせる必要があり、不適切な表示はなくなった。ただし、非加盟会社の製品は、インターネットを通じて販売されることが多く、未だに不適切な表示を継続する例が後を絶たない。

直管蛍光灯（FL40W形等）と同形状・同口金 (T8：G13) の物も発売され、LEDチップ価格の下落に伴い、ややコストメリットが出つつある。しかし、急速に価格が下落し、電球との消費電力の差も大きい「LED電球」と違い、直管蛍光灯型LEDは、もともと低消費電力の蛍光灯との競争のため、消費電力の差が少なく、価格も高い。カバーに透明と乳白色の2種類があり、直下の照度を重視するなら透明、広い照射角（最大310度のものもある）を求めるなら乳白色のものを選ぶのが妥当である。照明機器としてLED素子1個では充分な光束が得られないため、使用目的に合わせてLED素子を複数個使用して照度を確保している。100個以上のLED素子を使用した製品も珍しくない。ただし、蛍光灯に比べ重量が増すためにソケットが重みに耐えられず落下する危険性があるほか、蛍光灯器具の安定器を取り除く必要があるタイプのものも多い。そのため、日本の大手メーカーなどは器具そのものをLEDユニットにしたものを開発している。

丸形蛍光灯型LEDを使用するシーリングライト等についても、直管蛍光灯と同じく、もともと低消費電力の蛍光灯との競争のため、消費電力の差が少なく、価格も高い。

表面実装 (SMD) タイプのLEDを使用した照明器具を、「SMDライト」等と称して差別化して販売している例もあるが、本質的にLEDと何ら変わりがない。

乗用車のランプ

テールランプは、後続車両へのブレーキ作動の警告として使われる。そのため使用頻度が高く、急激な電力供給と発熱のため寿命が短い一方でランプ切れは事故につながりやすいため、長寿命のLEDが適している。また白熱型照明は発熱に時間がかかりそれがブレーキ作動から点灯までの時間差を生み事故の原因の一つになりうるが、LEDは時間差がきわめて少ない。

乗用車への利用も拡大しており、テールランプに加えアフターパーツとして室内灯やポジションランプ（スモールランプ）などが多く販売されている。光量が足りないためヘッドライトにLEDを採用例はなかったが、2007年5月発売の4代目LS600hには小糸製作所が日亜化学工業と共同開発した（鉄道以外の用途として）世界初のLEDヘッドランプが搭載されている。LS600hのLEDヘッドランプは1つのLEDランプでは光量は足りず3つのLEDランプをロービームとして使用していたが、その後LEDランプ1つあたりの光量が増え、2013年発売の3代目レクサスISでは1つのLEDランプでロービームとして使用できるようになった。LEDヘッドランプは消費電力が少なく光量はHIDより上回っており、各自動車メーカーが採用しつつある。

バイクなどのランプ

オートバイへの利用ではko-zaru仔猿（CKデザイン製）が、ウィンカーとテールランプ、ストップランプに2003年から採用している。小型バイクのためバッテリーの積載容量に制限があり、電力消費の点から採用した。日本では初めてのケースとなる。近年のLEDの性能向上を検証しつつ、ヘッドライトへのLEDの適用を研究している。一般市販バイク初搭載としては、ホンダが2014年3月14日発表、同月20日発売しているCB1300スーパーボルドール（型式SC54)の2014年モデルから正式採用された。

自転車のランプ

自転車用ランプのLED普及率は、自動車のそれに比べて非常に高い。発電機を動かすためペダルをこぐ力が乗り心地に直結するため、消費電力の少ないLEDの使用により軽快な乗り心地になる。また使用電力が低いため、非接触型の発電機を使用することにより、照明による負荷が非常に少なくなる。また電池式においても消費電力の少ない分電池が長持ちする利点がある。廉価な軽快車などでは相変わらず電球が主流であるが、ハブダイナモ式のオートライトには多く採用されている。この他、前照灯としての役目より、他の自転車や自動車からの被視認性を意識した認識灯や尾灯への応用も多い。

舞台演出用の照明器具として

高輝度LEDを搭載した舞台用照明器具がMARTIN社から発売されている。赤・青・緑（一部製品は白色）の高輝度LEDを搭載することにより一般的なフィラメントを用いた舞台照明と比較して次の利点が挙げられる。

• 消費電力が圧倒的に低い。
• 一つの照明につき多くの色を表現できる。シームレスな切り替えでグラデーションも可能である。

これらは一般的なフィラメント式のフレネル舞台照明よりも高価だが、舞台を始めコンサート・ライブ等で多く採用されている事例がある。

ガーデン ソーラーライト

構成

ガーテン用ソーラーライト

ソーラーパネルと充電式電池を使用するランプが普及して、各ホームセンターでは専用の売り場が設けられるまでになった。

電子写真式プリンター内部の感光用光源

電子写真式プリンターとして一般的なレーザープリンターは、レーザー光の出力を直接変化させたり、液晶シャッターで強度を変調した光を、回転するポリゴンミラー（多角形鏡）に反射させて走査したりして、感光ドラム上に走査線を作り出している。光学系には高い精度が要求され、構造上どうしてもある程度以上の走光路距離を確保せねばならず、プリンターの小型化、低価格化は困難だった。

これを解決したのが、LEDアレイヘッドを使用したLEDプリンターである。微細加工したLEDを直線上に数千 - 数万個並べ、感光ドラム上の潜像の1ドット1ドットに対応するLEDで感光書き込みを行う。機械的駆動系（ポリゴンミラー）は不要になり、光学系は単純な収束レンズのみで済み信頼性向上とコスト削減、機器の小型化を実現している。ただし、主走査解像度がヘッドの集積度によって制限される、素子間のばらつき補正が必要、ドラムとLEDアレイが非常に近いために飛散したトナーが付着して出力物のクオリティ安定性に欠けるなどの欠点も持つ。

光通信用光源

駆動電流の変化に対し、光出力が高速応答するという特性を生かし家電製品等の赤外線リモコンやTOSリンクを始めとする光ファイバー通信の信号送信機、またフォトカプラ内部の光源に赤外発光LEDが広く使われている。

模型製作・改造用光源

模型用点灯光源としても、価格低減と共にかつて使用されていた小型電球の代替として使用されるようになってきた。光色の制限から、かつては赤色光への使用が主だったが黄色、白色LEDの開発により前照灯や室内蛍光灯の白色光の再現も可能となった。さらに白熱灯の再現については電球色（淡橙色）LEDの開発により、実際の電球ではサイズや発熱などの理由で難しかった箇所も実感的な光色の再現が可能となった。特に、点灯機構を組み込むスペースが限られ、また部材がABSやポリスチレン樹脂などで作られているなど電球の発熱の面でも不利な場合があったNゲージを中心とした鉄道模型の場合、通常のレンズタイプからチップタイプへの移行により構造の小型化により実感の再現に大きく寄与し、これにより従来は実車のヘッドライト構造の関係で製品化が困難だった車種の製品化が実現した。コスト的には従来の電球使用より割高となっても実感的な模型の実現からユーザーに歓迎された面があり、分野としての消費量は少ないながらも実用照明器具での利用に先行して採用されている。また模型用途としては他にカーモデル用ディティールアップパーツやミニ四駆用のタミヤ純正カスタムパーツなど、改造用LEDキットが存在する。

エレクトロニックフラッシュやレフ板の代用として

カメラ(デジタルカメラも同様)では、暗所での撮影や接近撮影・人物撮影での際には露出のラティチュードを揃える意味でエレクトロニックフラッシュ(フラッシュ)やレフ板などを使って光を当てる事があるが、一般的なフラッシュ撮影では瞬間的に光を当てる撮影となるために、撮影者や被写体としては写真の仕上がりが想定しにくい。レフ板に関しては、自然な感じの照明効果が得られる半面、嵩ばる・移動の際に運搬がしにくい欠点がある。写真撮影用ライトは白熱電球の原理を用いたものが多いため、照明効率に対しての熱放射も大きく、被写体が熱を嫌う物である場合は照明器具として好ましくない例も多かった。またスタジオ外で撮影の為に携帯する機器は事実上、クリップオンフラッシュに限られた。LEDアレイ式ライトは電池での駆動が可能で、かつ照明光源としても必要十分な光量が得られるうえに比較的長時間の使用が可能なため、今後は撮影用照明器具としての普及が見込まれる。

2013年頃から、白色LEDをアレイ状に敷き詰めた撮影用LEDライトが、中国などを原生産国としてインターネットを中心に照明器具として普及しつつある。

ツェナーダイオードの代用品として

電子回路内の基準電圧源として一般に使われるツェナーダイオードはアバランシェ降伏現象を利用しているため、出力電圧にわずかながらノイズを発生させてしまう。通常はフィルタ回路によってノイズを十分に減衰させる設計を取るが、オペアンプをディスクリートで組む場合等、「そもそもノイズが発生しない基準電圧源」を追求して定電流駆動したLEDが使われる事例がある。

小信号ダイオードの代用品として

ディストーションやオーバードライブのクリッピング素子として、シリコンダイオードやゲルマニウムダイオードの代わりに使われる場合がある。

殺菌

深紫外線を発することにより水などを殺菌することができる。

関連項目

• 西澤潤一
• 中村修二
• ダブルヘテロ接合
• 半導体レーザー（レーザーダイオード）
• バンド理論
• pn接合
• エレクトロルミネセンス
• 有機エレクトロルミネッセンス（有機EL）
• 光起電力効果
• 電光掲示板
• フルカラー
• LED照明
• LED標識灯
• 高エネルギー可視光線, 青色光網膜傷害

wiki en

[Wiki en表示]

"LED" redirects here. For other uses, see LED (disambiguation).

Light-emitting diode

Blue, green, and red LEDs in 5 mm diffused case
Working principle	Electroluminescence
Invented	H. J. Round (1907) Oleg Losev (1927) James R. Biard (1961) Nick Holonyak (1962)
First production	October 1962
Pin configuration	Anode and cathode
Electronic symbol

Parts of a conventional LED. The flat bottom surfaces of the anvil and post embedded inside the epoxy act as anchors, to prevent the conductors from being forcefully pulled out via mechanical strain or vibration.

A bulb-shaped modern retrofit LED lamp with aluminium heat sink, a light diffusing dome and E27 screw base, using a built-in power supply working on mains voltage

Close up image of a surface mount LED

A light-emitting diode (LED) is a two-lead semiconductor light source. It is a p–n junction diode that emits light when activated. When a suitable voltage is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor. LEDs are typically small (less than 1 mm² ) and integrated optical components may be used to shape the radiation pattern.

Appearing as practical electronic components in 1962, the earliest LEDs emitted low-intensity infrared light. Infrared LEDs are still frequently used as transmitting elements in remote-control circuits, such as those in remote controls for a wide variety of consumer electronics. The first visible-light LEDs were also of low intensity and limited to red. Modern LEDs are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness.

Early LEDs were often used as indicator lamps for electronic devices, replacing small incandescent bulbs. They were soon packaged into numeric readouts in the form of seven-segment displays and were commonly seen in digital clocks. Recent developments have produced LEDs suitable for environmental and task lighting. LEDs have led to new displays and sensors, while their high switching rates are useful in advanced communications technology.

LEDs have many advantages over incandescent light sources, including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. Light-emitting diodes are used in applications as diverse as aviation lighting, automotive headlamps, advertising, general lighting, traffic signals, camera flashes, and lighted wallpaper. As of 2017, LED lights home room lighting are as cheap or cheaper than compact fluorescent lamp sources of comparable output. They are also significantly more energy efficient and, arguably, have fewer environmental concerns linked to their disposal.

History

Discoveries and early devices

Green electroluminescence from a point contact on a crystal of SiC recreates Round's original experiment from 1907.

Electroluminescence as a phenomenon was discovered in 1907 by the British experimenter H. J. Round of Marconi Labs, using a crystal of silicon carbide and a cat's-whisker detector. Russian inventor Oleg Losev reported creation of the first LED in 1927. His research was distributed in Soviet, German and British scientific journals, but no practical use was made of the discovery for several decades. Kurt Lehovec, Carl Accardo, and Edward Jamgochian explained these first light-emitting diodes in 1951 using an apparatus employing SiC crystals with a current source of battery or pulse generator and with a comparison to a variant, pure, crystal in 1953.

Rubin Braunstein of the Radio Corporation of America reported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys in 1955. Braunstein observed infrared emission generated by simple diode structures using gallium antimonide (GaSb), GaAs, indium phosphide (InP), and silicon-germanium (SiGe) alloys at room temperature and at 77 Kelvin.

In 1957, Braunstein further demonstrated that the rudimentary devices could be used for non-radio communication across a short distance. As noted by Kroemer Braunstein "…had set up a simple optical communications link: Music emerging from a record player was used via suitable electronics to modulate the forward current of a GaAs diode. The emitted light was detected by a PbS diode some distance away. This signal was fed into an audio amplifier and played back by a loudspeaker. Intercepting the beam stopped the music. We had a great deal of fun playing with this setup." This setup presaged the use of LEDs for optical communication applications.

A Texas Instruments SNX-100 GaAs LED contained in a TO-18 transistor metal case.

In September 1961, while working at Texas Instruments in Dallas, Texas, James R. Biard and Gary Pittman discovered near-infrared (900 nm) light emission from a tunnel diode they had constructed on a GaAs substrate. By October 1961, they had demonstrated efficient light emission and signal coupling between a GaAs p-n junction light emitter and an electrically-isolated semiconductor photodetector. On August 8, 1962, Biard and Pittman filed a patent titled "Semiconductor Radiant Diode" based on their findings, which described a zinc diffused p–n junction LED with a spaced cathode contact to allow for efficient emission of infrared light under forward bias. After establishing the priority of their work based on engineering notebooks predating submissions from G.E. Labs, RCA Research Labs, IBM Research Labs, Bell Labs, and Lincoln Lab at MIT, the U.S. patent office issued the two inventors the patent for the GaAs infrared (IR) light-emitting diode (U.S. Patent US3293513), the first practical LED. Immediately after filing the patent, Texas Instruments (TI) began a project to manufacture infrared diodes. In October 1962, TI announced the first commercial LED product (the SNX-100), which employed a pure GaAs crystal to emit a 890 nm light output. In October 1963, TI announced the first commercial hemispherical LED, the SNX-110.

The first visible-spectrum (red) LED was developed in 1962 by Nick Holonyak, Jr. while working at General Electric. Holonyak first reported his LED in the journal Applied Physics Letters on December 1, 1962. M. George Craford, a former graduate student of Holonyak, invented the first yellow LED and improved the brightness of red and red-orange LEDs by a factor of ten in 1972. In 1976, T. P. Pearsall created the first high-brightness, high-efficiency LEDs for optical fiber telecommunications by inventing new semiconductor materials specifically adapted to optical fiber transmission wavelengths.

Initial commercial development

The first commercial LEDs were commonly used as replacements for incandescent and neon indicator lamps, and in seven-segment displays, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios, telephones, calculators, as well as watches (see list of signal uses). Until 1968, visible and infrared LEDs were extremely costly, in the order of US$200 per unit, and so had little practical use. The Monsanto Company was the first organization to mass-produce visible LEDs, using gallium arsenide phosphide (GaAsP) in 1968 to produce red LEDs suitable for indicators. Hewlett-Packard (HP) introduced LEDs in 1968, initially using GaAsP supplied by Monsanto. These red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Readouts in calculators were so small that plastic lenses were built over each digit to make them legible. Later, other colors became widely available and appeared in appliances and equipment. In the 1970s commercially successful LED devices at less than five cents each were produced by Fairchild Optoelectronics. These devices employed compound semiconductor chips fabricated with the planar process invented by Dr. Jean Hoerni at Fairchild Semiconductor. The combination of planar processing for chip fabrication and innovative packaging methods enabled the team at Fairchild led by optoelectronics pioneer Thomas Brandt to achieve the needed cost reductions. LED producers continue to use these methods.

LED display of a TI-30 scientific calculator (ca. 1978), which uses plastic lenses to increase the visible digit size

Most LEDs were made in the very common 5 mm T1¾ and 3 mm T1 packages, but with rising power output, it has grown increasingly necessary to shed excess heat to maintain reliability, so more complex packages have been adapted for efficient heat dissipation. Packages for state-of-the-art high-power LEDs bear little resemblance to early LEDs.

Blue LED

Blue LEDs were first developed by Herbert Paul Maruska at RCA in 1972 using gallium nitride (GaN) on a sapphire substrate. SiC-types were first commercially sold in the United States by Cree in 1989. However, neither of these initial blue LEDs were very bright.

The first high-brightness blue LED was demonstrated by Shuji Nakamura of Nichia Corporation in 1994 and was based on InGaN. In parallel, Isamu Akasaki and Hiroshi Amano in Nagoya were working on developing the important GaN nucleation on sapphire substrates and the demonstration of p-type doping of GaN. Nakamura, Akasaki, and Amano were awarded the 2014 Nobel prize in physics for their work. In 1995, Alberto Barbieri at the Cardiff University Laboratory (GB) investigated the efficiency and reliability of high-brightness LEDs and demonstrated a "transparent contact" LED using indium tin oxide (ITO) on (AlGaInP/GaAs).

In 2001 and 2002, processes for growing gallium nitride (GaN) LEDs on silicon were successfully demonstrated. In January 2012, Osram demonstrated high-power InGaN LEDs grown on silicon substrates commercially.

White LEDs and the illumination breakthrough

The attainment of high efficiency in blue LEDs was quickly followed by the development of the first white LED. In this device a Y
3Al
5O
12:Ce (known as "YAG") phosphor coating on the emitter absorbs some of the blue emission and produces yellow light through fluorescence. The combination of that yellow with remaining blue light appears white to the eye. However, using different phosphors (fluorescent materials) it also became possible to instead produce green and red light through fluorescence. The resulting mixture of red, green and blue is not only perceived by humans as white light but is superior for illumination in terms of color rendering, whereas one cannot appreciate the color of red or green objects illuminated only by the yellow (and remaining blue) wavelengths from the YAG phosphor.

Illustration of Haitz's law, showing improvement in light output per LED over time, with a logarithmic scale on the vertical axis

The first white LEDs were expensive and inefficient. However, the light output of LEDs has increased exponentially, with a doubling occurring approximately every 36 months since the 1960s (similar to Moore's law). This trend is generally attributed to the parallel development of other semiconductor technologies and advances in optics and materials science and has been called Haitz's law after Dr. Roland Haitz.

Light output and efficiency of blue and near-ultraviolet LEDs rose as the cost of reliable devices fell. This led to relatively high-power white-light LEDs for illumination, which are replacing incandescent and fluorescent lighting.

Experimental white LEDs have been demonstrated to produce over 300 lumens per watt of electricity; some can last up to 100,000 hours. Compared to incandescent bulbs, this is not only a huge increase in electrical efficiency but – over time – a similar or lower cost per bulb.

Working principle

The inner workings of an LED, showing circuit (top) and band diagram (bottom)

A P-N junction can convert absorbed light energy into a proportional electric current. The same process is reversed here (i.e. the P-N junction emits light when electrical energy is applied to it). This phenomenon is generally called electroluminescence, which can be defined as the emission of light from a semiconductor under the influence of an electric field. The charge carriers recombine in a forward-biased P-N junction as the electrons cross from the N-region and recombine with the holes existing in the P-region. Free electrons are in the conduction band of energy levels, while holes are in the valence energy band. Thus the energy level of the holes is less than the energy levels of the electrons. Some portion of the energy must be dissipated to recombine the electrons and the holes. This energy is emitted in the form of heat and light.

The electrons dissipate energy in the form of heat for silicon and germanium diodes but in gallium arsenide phosphide (GaAsP) and gallium phosphide (GaP) semiconductors, the electrons dissipate energy by emitting photons. If the semiconductor is translucent, the junction becomes the source of light as it is emitted, thus becoming a light-emitting diode. However, when the junction is reverse biased, the LED produces no light and—if the potential is great enough, the device is damaged.

Technology

I-V diagram for a diode. An LED begins to emit light when more than 2 or 3 volts is applied. The reverse bias region uses a different vertical scale from the forward bias region to show that the leakage current is nearly constant with voltage until breakdown occurs. In forward bias, the current is small but increases exponentially with voltage.

Physics

The LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level and releases energy in the form of a photon.

The wavelength of the light emitted, and thus its color, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes usually recombine by a non-radiative transition, which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible, or near-ultraviolet light.

LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have enabled making devices with ever-shorter wavelengths, emitting light in a variety of colors.

LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.

Refractive index

Idealized example of light emission cones in a simple square semiconductor, for a single point-source emission zone. The left illustration is for a translucent wafer, while the right illustration shows the half-cones formed when the bottom layer is opaque. The light is actually emitted equally in all directions from the point-source, but can only escape perpendicular to the semiconductor's surface and some degrees to the side, which is illustrated by the cone shapes. When the critical angle is exceeded, photons are reflected internally. The areas between the cones represent the trapped light energy wasted as heat.

Most materials used for LED production have very high refractive indices. This means that much of the light is reflected back into the material at the material/air surface interface. Thus, light extraction in LEDs is an important aspect of LED production, subject to much research and development. The light emission cones of a real LED wafer are far more complex than a single point-source light emission. The light emission zone is typically a two-dimensional plane between the wafers. Every atom across this plane has an individual set of emission cones. Drawing the billions of overlapping cones is impossible, so this is a simplified diagram showing the extents of all the emission cones combined. The larger side cones are clipped to show the interior features and reduce image complexity; they would extend to the opposite edges of the two-dimensional emission plane.

Bare uncoated semiconductors such as silicon exhibit a very high refractive index relative to open air, which prevents passage of photons arriving at sharp angles relative to the air-contacting surface of the semiconductor due to total internal reflection. This property affects both the light-emission efficiency of LEDs as well as the light-absorption efficiency of photovoltaic cells. The refractive index of silicon is 3.96 (at 590 nm), while air is 1.0002926.

In general, a flat-surface uncoated LED semiconductor chip emits light only perpendicular to the semiconductor's surface, and a few degrees to the side, in a cone shape referred to as the light cone, cone of light, or the escape cone. The maximum angle of incidence is referred to as the critical angle. When this angle is exceeded, photons no longer escape the semiconductor but are instead reflected internally inside the semiconductor crystal as if it were a mirror.

Internal reflections can escape through other crystalline faces if the incidence angle is low enough and the crystal is sufficiently transparent to not re-absorb the photon emission. But for a simple square LED with 90-degree angled surfaces on all sides, the faces all act as equal angle mirrors. In this case, most of the light can not escape and is lost as waste heat in the crystal.

A convoluted chip surface with angled facets similar to a jewel or fresnel lens can increase light output by distributing light perpendicular to the chip surface and far to the sides of the photon emission point.

The ideal shape of a semiconductor with maximum light output would be a microsphere with the photon emission occurring at the exact center, with electrodes penetrating to the center to contact at the emission point. All light rays emanating from the center would be perpendicular to the entire surface of the sphere, resulting in no internal reflections. A hemispherical semiconductor would also work, with the flat back-surface serving as a mirror to back-scattered photons.

Transition coatings

After the doping of the wafer, it is cut apart into individual dies. Each die is commonly called a chip.

Many LED semiconductor chips are encapsulated or potted in clear or colored molded plastic shells. The plastic shell has three purposes:

1. Mounting the semiconductor chip in devices is easier to accomplish.
2. The tiny fragile electrical wiring is physically supported and protected from damage.
3. The plastic acts as a refractive intermediary between the relatively high-index semiconductor and low-index open air.

The third feature helps to boost the light emission from the semiconductor by acting as a diffusing lens, emitting light at a much higher angle of incidence from the light cone than the bare chip would alone.

Efficiency and operational parameters

Typical indicator LEDs are designed to operate with no more than 30–60 milliwatts (mW) of electrical power. Around 1999, Philips Lumileds introduced power LEDs capable of continuous use at one watt. These LEDs used much larger semiconductor die sizes to handle the large power inputs. Also, the semiconductor dies were mounted onto metal slugs to allow for heat removal from the LED die.

One of the key advantages of LED-based lighting sources is high luminous efficacy. White LEDs quickly matched and overtook the efficacy of standard incandescent lighting systems. In 2002, Lumileds made five-watt LEDs available with luminous efficacy of 18–22 lumens per watt (lm/W). For comparison, a conventional incandescent light bulb of 60–100 watts emits around 15 lm/W, and standard fluorescent lights emit up to 100 lm/W.

As of 2012, Philips had achieved the following efficacies for each color. The efficiency values show the physics – light power out per electrical power in. The lumen-per-watt efficacy value includes characteristics of the human eye and is derived using the luminosity function.

	Color	Wavelength range (nm)	Typical efficiency coefficient	Typical efficacy (lm/W)
	Red	620 < λ < 645	0.39	72
	Red-orange	610 < λ < 620	0.29	98
	Green	520 < λ < 550	0.15	93
	Cyan	490 < λ < 520	0.26	75
	Blue	460 < λ < 490	0.35	37

In September 2003, a new type of blue LED was demonstrated by Cree that consumes 24 mW at 20 milliamperes (mA). This produced a commercially packaged white light giving 65 lm/W at 20 mA, becoming the brightest white LED commercially available at the time, and more than four times as efficient as standard incandescents. In 2006, they demonstrated a prototype with a record white LED luminous efficacy of 131 lm/W at 20 mA. Nichia Corporation has developed a white LED with luminous efficacy of 150 lm/W at a forward current of 20 mA. Cree's XLamp XM-L LEDs, commercially available in 2011, produce 100 lm/W at their full power of 10 W, and up to 160 lm/W at around 2 W input power. In 2012, Cree announced a white LED giving 254 lm/W, and 303 lm/W in March 2014. Practical general lighting needs high-power LEDs, of one watt or more. Typical operating currents for such devices begin at 350 mA.

These efficiencies are for the light-emitting diode only, held at low temperature in a lab. Since LEDs installed in real fixtures operate at higher temperature and with driver losses, real-world efficiencies are much lower. United States Department of Energy (DOE) testing of commercial LED lamps designed to replace incandescent lamps or CFLs showed that average efficacy was still about 46 lm/W in 2009 (tested performance ranged from 17 lm/W to 79 lm/W).

Efficiency droop

Efficiency droop is the decrease in luminous efficiency of LEDs as the electric current increases above tens of milliamperes.

This effect was initially theorized to be related to elevated temperatures. Scientists proved the opposite is true: though the life of an LED would be shortened, the efficiency droop is less severe at elevated temperatures. The mechanism causing efficiency droop was identified in 2007 as Auger recombination, which was taken with mixed reaction. In 2013, a study confirmed Auger recombination as the cause of efficiency droop.

In addition to being less efficient, operating LEDs at higher electric currents creates higher heat levels, which compromise LED lifetime. Because of this increased heat at higher currents, high-brightness LEDs have an industry standard of operating at only 350 mA, which is a compromise between light output, efficiency, and longevity.

Possible solutions

Instead of increasing current levels, luminance is usually increased by combining multiple LEDs in one bulb. Solving the problem of efficiency droop would mean that household LED light bulbs would need fewer LEDs, which would significantly reduce costs.

Researchers at the U.S. Naval Research Laboratory have found a way to lessen the efficiency droop. They found that the droop arises from non-radiative Auger recombination of the injected carriers. They created quantum wells with a soft confinement potential to lessen the non-radiative Auger processes.

Researchers at Taiwan National Central University and Epistar Corp are developing a way to lessen the efficiency droop by using ceramic aluminium nitride (AlN) substrates, which are more thermally conductive than the commercially used sapphire. The higher thermal conductivity reduces self-heating effects.

Lifetime and failure

Main article: List of LED failure modes

Solid-state devices such as LEDs are subject to very limited wear and tear if operated at low currents and at low temperatures. Typical lifetimes quoted are 25,000 to 100,000 hours, but heat and current settings can extend or shorten this time significantly.

The most common symptom of LED (and diode laser) failure is the gradual lowering of light output and loss of efficiency. Sudden failures, although rare, can also occur. Early red LEDs were notable for their short service life. With the development of high-power LEDs, the devices are subjected to higher junction temperatures and higher current densities than traditional devices. This causes stress on the material and may cause early light-output degradation. To quantitify useful lifetime in a standardized manner, some suggest using L70 or L50, which are runtimes (typically in thousands of hours) at which a given LED reaches 70% and 50% of initial light output, respectively.

Whereas in most previous sources of light (incandescent lamps, discharge lamps, and those that burn combustible fuel, e.g. candles and oil lamps) the light results from heat, LEDs only operate if they are kept cool enough. The manufacturer commonly specifies a maximum junction temperature of 125 or 150 °C, and lower temperatures are advisable in the interests of long life. At these temperatures, relatively little heat is lost by radiation, which means that the light beam generated by an LED is cool.

The waste heat in a high-power LED (which as of 2015 can be less than half the power that it consumes) is conveyed by conduction through the substrate and package of the LED to a heat sink, which gives up the heat to the ambient air by convection. Careful thermal design is, therefore, essential, taking into account the thermal resistances of the LED’s package, the heat sink and the interface between the two. Medium-power LEDs are often designed to solder directly to a printed circuit board that contains a thermally conductive metal layer. High-power LEDs are packaged in large-area ceramic packages that attach to a metal heat sink—the interface being a material with high thermal conductivity (thermal grease, phase-change material, thermally conductive pad, or thermal adhesive).

If an LED-based lamp is installed in an unventilated luminaire, or a luminaire is located in an environment that does not have free air circulation, the LED is likely to overheat, resulting in reduced life or early catastrophic failure. Thermal design is often based on an ambient temperature of 25 °C (77 °F). LEDs used in outdoor applications, such as traffic signals or in-pavement signal lights, and in climates where the temperature within the light fixture gets very high, could experience reduced output or even failure.

Since LED efficacy is higher at low temperatures, LED technology is well suited for supermarket freezer lighting. Because LEDs produce less waste heat than incandescent lamps, their use in freezers can save on refrigeration costs as well. However, they may be more susceptible to frost and snow buildup than incandescent lamps, so some LED lighting systems have been designed with an added heating circuit. Additionally, research has developed heat sink technologies that transfer heat produced within the junction to appropriate areas of the light fixture.

Colors and materials

Conventional LEDs are made from a variety of inorganic semiconductor materials. The following table shows the available colors with wavelength range, voltage drop, and material:

	Color	Wavelength [nm]	Voltage drop [ΔV]	Semiconductor material
	Infrared	λ > 760	ΔV < 1.63	Gallium arsenide (GaAs) Aluminium gallium arsenide (AlGaAs)
	Red	610 < λ < 760	1.63 < ΔV < 2.03	Aluminium gallium arsenide (AlGaAs) Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)
	Orange	590 < λ < 610	2.03 < ΔV < 2.10	Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)
	Yellow	570 < λ < 590	2.10 < ΔV < 2.18	Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)
	Green	500 < λ < 570	1.9 < ΔV < 4.0	Traditional green: Gallium(III) phosphide (GaP) Aluminium gallium indium phosphide (AlGaInP) Aluminium gallium phosphide (AlGaP) Pure green: Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN)
	Blue	450 < λ < 500	2.48 < ΔV < 3.7	Zinc selenide (ZnSe) Indium gallium nitride (InGaN) Silicon carbide (SiC) as substrate Silicon (Si) as substrate—under development
	Violet	400 < λ < 450	2.76 < ΔV < 4.0	Indium gallium nitride (InGaN)
	Purple	Multiple types	2.48 < ΔV < 3.7	Dual blue/red LEDs, blue with red phosphor, or white with purple plastic
	Ultraviolet	λ < 400	3 < ΔV < 4.1	Indium gallium nitride (InGaN) (385-400 nm) Diamond (235 nm) Boron nitride (215 nm) Aluminium nitride (AlN) (210 nm) Aluminium gallium nitride (AlGaN) Aluminium gallium indium nitride (AlGaInN)—down to 210 nm
	Pink	Multiple types	ΔV ≈3.3	Blue with one or two phosphor layers, yellow with red, orange or pink phosphor added afterwards, white with pink plastic, or white phosphors with pink pigment or dye over top.
	White	Broad spectrum	2.8 < ΔV < 4.2	Cool / Pure White: Blue/UV diode with yellow phosphor Warm White: Blue diode with orange phosphor

Blue and ultraviolet

Blue LEDs

External video

“The Original Blue LED”, Chemical Heritage Foundation

The first blue-violet LED using magnesium-doped gallium nitride was made at Stanford University in 1972 by Herb Maruska and Wally Rhines, doctoral students in materials science and engineering. At the time Maruska was on leave from RCA Laboratories, where he collaborated with Jacques Pankove on related work. In 1971, the year after Maruska left for Stanford, his RCA colleagues Pankove and Ed Miller demonstrated the first blue electroluminescence from zinc-doped gallium nitride, though the subsequent device Pankove and Miller built, the first actual gallium nitride light-emitting diode, emitted green light. In 1974 the U.S. Patent Office awarded Maruska, Rhines and Stanford professor David Stevenson a patent for their work in 1972 (U.S. Patent US3819974 A) and today, magnesium-doping of gallium nitride remains the basis for all commercial blue LEDs and laser diodes. In the early 1970s, these devices were too dim for practical use, and research into gallium nitride devices slowed. In August 1989, Cree introduced the first commercially available blue LED based on the indirect bandgap semiconductor, silicon carbide (SiC). SiC LEDs had very low efficiency, no more than about 0.03%, but did emit in the blue portion of the visible light spectrum.

In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping ushered in the modern era of GaN-based optoelectronic devices. Building upon this foundation, Theodore Moustakas at Boston University patented a method for producing high-brightness blue LEDs using a new two-step process. Two years later, in 1993, high-brightness blue LEDs were demonstrated again by Shuji Nakamura of Nichia Corporation using a gallium nitride growth process similar to Moustakas's. Both Moustakas and Nakamura were issued separate patents, which confused the issue of who was the original inventor (partly because although Moustakas invented his first, Nakamura filed first). This new development revolutionized LED lighting, making high-power blue light sources practical, leading to the development of technologies like Blu-ray, as well as allowing the bright high-resolution screens of modern tablets and phones.

Nakamura was awarded the 2006 Millennium Technology Prize for his invention. Nakamura, Hiroshi Amano and Isamu Akasaki were awarded the Nobel Prize in Physics in 2014 for the invention of the blue LED. In 2015, a US court ruled that three companies (i.e. the litigants who had not previously settled out of court) that had licensed Nakamura's patents for production in the United States had infringed Moustakas's prior patent, and ordered them to pay licensing fees of not less than 13 million USD.

By the late 1990s, blue LEDs became widely available. They have an active region consisting of one or more InGaN quantum wells sandwiched between thicker layers of GaN, called cladding layers. By varying the relative In/Ga fraction in the InGaN quantum wells, the light emission can in theory be varied from violet to amber. Aluminium gallium nitride (AlGaN) of varying Al/Ga fraction can be used to manufacture the cladding and quantum well layers for ultraviolet LEDs, but these devices have not yet reached the level of efficiency and technological maturity of InGaN/GaN blue/green devices. If un-alloyed GaN is used in this case to form the active quantum well layers, the device emits near-ultraviolet light with a peak wavelength centred around 365 nm. Green LEDs manufactured from the InGaN/GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems, but practical devices still exhibit efficiency too low for high-brightness applications.

With nitrides containing aluminium, most often AlGaN and AlGaInN, even shorter wavelengths are achievable. Ultraviolet LEDs in a range of wavelengths are becoming available on the market. Near-UV emitters at wavelengths around 375–395 nm are already cheap and often encountered, for example, as black light lamp replacements for inspection of anti-counterfeiting UV watermarks in some documents and paper currencies. Shorter-wavelength diodes, while substantially more expensive, are commercially available for wavelengths down to 240 nm. As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with a peak at about 260 nm, UV LED emitting at 250–270 nm are to be expected in prospective disinfection and sterilization devices. Recent research has shown that commercially available UVA LEDs (365 nm) are already effective disinfection and sterilization devices. UV-C wavelengths were obtained in laboratories using aluminium nitride (210 nm), boron nitride (215 nm) and diamond (235 nm).

RGB

RGB-SMD-LED

RGB LEDs consist of one red, one green, and one blue LED. By independently adjusting each of the three, RGB LEDs are capable of producing a wide color gamut. Unlike dedicated-color LEDs, however, these obviously do not produce pure wavelengths. Moreover, such modules as commercially available are often not optimized for smooth color mixing.

White

There are two primary ways of producing white light-emitting diodes (WLEDs), LEDs that generate high-intensity white light. One is to use individual LEDs that emit three primary colors—red, green, and blue—and then mix all the colors to form white light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, much in the same way a fluorescent light bulb works. It is important to note that the 'whiteness' of the light produced is essentially engineered to suit the human eye, and depending on the situation it may not always be appropriate to think of it as white light.

There are three main methods of mixing colors to produce white light from an LED:

• blue LED + green LED + red LED (color mixing; can be used as backlighting for displays, extremely poor for illumination due to gaps in spectrum)
• near-UV or UV LED + RGB phosphor (an LED producing light with a wavelength shorter than blue's is used to excite an RGB phosphor)
• blue LED + yellow phosphor (two complementary colors combine to form white light; more efficient than first two methods and more commonly used)

Because of metamerism, it is possible to have quite different spectra that appear white. However, the appearance of objects illuminated by that light may vary as the spectrum varies, this is the issue of Colour rendition, quite separate from Colour Temperature, where a really orange or cyan object could appear with the wrong colour and much darker as the LED or phosphor does not emit the wavelength. The best colour rendition CFL and LEDs use a mix of phosphors, resulting in less efficiency but better quality of light. Though incandescent halogen lamps have a more orange colour temperature, they are still the best easily available artificial light sources in terms of colour rendition.

RGB systems

Combined spectral curves for blue, yellow-green, and high-brightness red solid-state semiconductor LEDs. FWHM spectral bandwidth is approximately 24–27 nm for all three colors.

RGB LED

White light can be formed by mixing differently colored lights; the most common method is to use red, green, and blue (RGB). Hence the method is called multi-color white LEDs (sometimes referred to as RGB LEDs). Because these need electronic circuits to control the blending and diffusion of different colors, and because the individual color LEDs typically have slightly different emission patterns (leading to variation of the color depending on direction) even if they are made as a single unit, these are seldom used to produce white lighting. Nonetheless, this method has many applications because of the flexibility of mixing different colors, and in principle, this mechanism also has higher quantum efficiency in producing white light.

There are several types of multi-color white LEDs: di-, tri-, and tetrachromatic white LEDs. Several key factors that play among these different methods include color stability, color rendering capability, and luminous efficacy. Often, higher efficiency means lower color rendering, presenting a trade-off between the luminous efficacy and color rendering. For example, the dichromatic white LEDs have the best luminous efficacy (120 lm/W), but the lowest color rendering capability. However, although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficacy. Trichromatic white LEDs are in between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability.

One of the challenges is the development of more efficient green LEDs. The theoretical maximum for green LEDs is 683 lumens per watt but as of 2010 few green LEDs exceed even 100 lumens per watt. The blue and red LEDs get closer to their theoretical limits.

Multi-color LEDs offer not merely another means to form white light but a new means to form light of different colors. Most perceivable colors can be formed by mixing different amounts of three primary colors. This allows precise dynamic color control. As more effort is devoted to investigating this method, multi-color LEDs should have profound influence on the fundamental method that we use to produce and control light color. However, before this type of LED can play a role on the market, several technical problems must be solved. These include that this type of LED's emission power decays exponentially with rising temperature, resulting in a substantial change in color stability. Such problems inhibit and may preclude industrial use. Thus, many new package designs aimed at solving this problem have been proposed and their results are now being reproduced by researchers and scientists. However multi-colour LEDs without phosphors can never provide good quality lighting because each LED is a narrow band source (see graph). LEDs without phosphor while a poorer solution for general lighting are the best solution for displays, either backlight of LCD, or direct LED based pixels.

Correlated color temperature (CCT) dimming for LED technology is regarded as a difficult task since binning, age and temperature drift effects of LEDs change the actual color value output. Feedback loop systems are used for example with color sensors, to actively monitor and control the color output of multiple color mixing LEDs.

Phosphor-based LEDs

Spectrum of a white LED showing blue light directly emitted by the GaN-based LED (peak at about 465 nm) and the more broadband Stokes-shifted light emitted by the Ce³⁺:YAG phosphor, which emits at roughly 500–700 nm

This method involves coating LEDs of one color (mostly blue LEDs made of InGaN) with phosphors of different colors to form white light; the resultant LEDs are called phosphor-based or phosphor-converted white LEDs (pcLEDs). A fraction of the blue light undergoes the Stokes shift being transformed from shorter wavelengths to longer. Depending on the color of the original LED, phosphors of different colors can be employed. If several phosphor layers of distinct colors are applied, the emitted spectrum is broadened, effectively raising the color rendering index (CRI) value of a given LED.

Phosphor-based LED efficiency losses are due to the heat loss from the Stokes shift and also other phosphor-related degradation issues. Their luminous efficacies compared to normal LEDs depend on the spectral distribution of the resultant light output and the original wavelength of the LED itself. For example, the luminous efficacy of a typical YAG yellow phosphor based white LED ranges from 3 to 5 times the luminous efficacy of the original blue LED because of the human eye's greater sensitivity to yellow than to blue (as modeled in the luminosity function). Due to the simplicity of manufacturing, the phosphor method is still the most popular method for making high-intensity white LEDs. The design and production of a light source or light fixture using a monochrome emitter with phosphor conversion is simpler and cheaper than a complex RGB system, and the majority of high-intensity white LEDs presently on the market are manufactured using phosphor light conversion.

Among the challenges being faced to improve the efficiency of LED-based white light sources is the development of more efficient phosphors. As of 2010, the most efficient yellow phosphor is still the YAG phosphor, with less than 10% Stokes shift loss. Losses attributable to internal optical losses due to re-absorption in the LED chip and in the LED packaging itself account typically for another 10% to 30% of efficiency loss. Currently, in the area of phosphor LED development, much effort is being spent on optimizing these devices to higher light output and higher operation temperatures. For instance, the efficiency can be raised by adapting better package design or by using a more suitable type of phosphor. Conformal coating process is frequently used to address the issue of varying phosphor thickness.

Some phosphor-based white LEDs encapsulate InGaN blue LEDs inside phosphor-coated epoxy. Alternatively, the LED might be paired with a remote phosphor, a preformed polycarbonate piece coated with the phosphor material. Remote phosphors provide more diffuse light, which is desirable for many applications. Remote phosphor designs are also more tolerant of variations in the LED emissions spectrum. A common yellow phosphor material is cerium-doped yttrium aluminium garnet (Ce³⁺:YAG).

White LEDs can also be made by coating near-ultraviolet (NUV) LEDs with a mixture of high-efficiency europium-based phosphors that emit red and blue, plus copper and aluminium-doped zinc sulfide (ZnS:Cu, Al) that emits green. This is a method analogous to the way fluorescent lamps work. This method is less efficient than blue LEDs with YAG:Ce phosphor, as the Stokes shift is larger, so more energy is converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both methods offer comparable brightness. A concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin.

Other white LEDs

Another method used to produce experimental white light LEDs used no phosphors at all and was based on homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate that simultaneously emitted blue light from its active region and yellow light from the substrate.

A new style of wafers composed of gallium-nitride-on-silicon (GaN-on-Si) is being used to produce white LEDs using 200-mm silicon wafers. This avoids the typical costly sapphire substrate in relatively small 100- or 150-mm wafer sizes. The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted. It is predicted that by 2020, 40% of all GaN LEDs will be made with GaN-on-Si. Manufacturing large sapphire material is difficult, while large silicon material is cheaper and more abundant. LED companies shifting from using sapphire to silicon should be a minimal investment.

Organic light-emitting diodes (OLEDs)

Main article: Organic light-emitting diode

Orange light-emitting diode

In an organic light-emitting diode (OLED), the electroluminescent material comprising the emissive layer of the diode is an organic compound. The organic material is electrically conductive due to the delocalization of pi electrons caused by conjugation over all or part of the molecule, and the material therefore functions as an organic semiconductor. The organic materials can be small organic molecules in a crystalline phase, or polymers.

The potential advantages of OLEDs include thin, low-cost displays with a low driving voltage, wide viewing angle, and high contrast and color gamut. Polymer LEDs have the added benefit of printable and flexible displays. OLEDs have been used to make visual displays for portable electronic devices such as cellphones, digital cameras, and MP3 players while possible future uses include lighting and televisions.

Quantum dot LEDs

See also: quantum dot display

Quantum dots (QD) are semiconductor nanocrystals with optical properties that let their emission color be tuned from the visible into the infrared spectrum. This allows quantum dot LEDs to create almost any color on the CIE diagram. This provides more color options and better color rendering than white LEDs since the emission spectrum is much narrower, characteristic of quantum confined states.

There are two types of schemes for QD excitation. One uses photo excitation with a primary light source LED (typically blue or UV LEDs are used). The other is direct electrical excitation first demonstrated by Alivisatos et al.

One example of the photo-excitation scheme is a method developed by Michael Bowers, at Vanderbilt University in Nashville, involving coating a blue LED with quantum dots that glow white in response to the blue light from the LED. This method emits a warm, yellowish-white light similar to that made by incandescent light bulbs. Quantum dots are also being considered for use in white light-emitting diodes in liquid crystal display (LCD) televisions.

In February 2011 scientists at PlasmaChem GmbH were able to synthesize quantum dots for LED applications and build a light converter on their basis, which was able to efficiently convert light from blue to any other color for many hundred hours. Such QDs can be used to emit visible or near infrared light of any wavelength being excited by light with a shorter wavelength.

The structure of QD-LEDs used for the electrical-excitation scheme is similar to basic design of OLEDs. A layer of quantum dots is sandwiched between layers of electron-transporting and hole-transporting materials. An applied electric field causes electrons and holes to move into the quantum dot layer and recombine forming an exciton that excites a QD. This scheme is commonly studied for quantum dot display. The tunability of emission wavelengths and narrow bandwidth is also beneficial as excitation sources for fluorescence imaging. Fluorescence near-field scanning optical microscopy (NSOM) utilizing an integrated QD-LED has been demonstrated.

In February 2008, a luminous efficacy of 300 lumens of visible light per watt of radiation (not per electrical watt) and warm-light emission was achieved by using nanocrystals.

Types

LEDs are produced in a variety of shapes and sizes. The color of the plastic lens is often the same as the actual color of light emitted, but not always. For instance, purple plastic is often used for infrared LEDs, and most blue devices have colorless housings. Modern high-power LEDs such as those used for lighting and backlighting are generally found in surface-mount technology (SMT) packages (not shown).

The main types of LEDs are miniature, high-power devices and custom designs such as alphanumeric or multi-color.

Miniature

Photo of miniature surface mount LEDs in most common sizes. They can be much smaller than a traditional 5 mm lamp type LED, shown on the upper left corner.

Very small (1.6x1.6x0.35 mm) red, green, and blue surface mount miniature LED package with gold wire bonding details.

These are mostly single-die LEDs used as indicators, and they come in various sizes from 2 mm to 8 mm, through-hole and surface mount packages. They usually do not use a separate heat sink. Typical current ratings range from around 1 mA to above 20 mA. The small size sets a natural upper boundary on power consumption due to heat caused by the high current density and need for a heat sink. Often daisy chained as used in LED tapes.

Common package shapes include round, with a domed or flat top, rectangular with a flat top (as used in bar-graph displays), and triangular or square with a flat top. The encapsulation may also be clear or tinted to improve contrast and viewing angle.

Researchers at the University of Washington have invented the thinnest LED. It is made of two-dimensional (2-D) flexible materials. It is three atoms thick, which is 10 to 20 times thinner than three-dimensional (3-D) LEDs and is also 10,000 times smaller than the thickness of a human hair. These 2-D LEDs are going to make it possible to create smaller, more energy-efficient lighting, optical communication and nano lasers.

There are three main categories of miniature single die LEDs:

Low-current

Typically rated for 2 mA at around 2 V (approximately 4 mW consumption)

Standard

20 mA LEDs (ranging from approximately 40 mW to 90 mW) at around:

• 1.9 to 2.1 V for red, orange, yellow, and traditional green
• 3.0 to 3.4 V for pure green and blue
• 2.9 to 4.2 V for violet, pink, purple and white

Ultra-high-output

20 mA at approximately 2 or 4–5 V, designed for viewing in direct sunlight

5 V and 12 V LEDs are ordinary miniature LEDs that incorporate a suitable series resistor for direct connection to a 5 V or 12 V supply.

High-power

High-power light-emitting diodes attached to an LED star base (Luxeon, Lumileds)

See also: Solid-state lighting, LED lamp, and Thermal management of high-power LEDs

High-power LEDs (HP-LEDs) or high-output LEDs (HO-LEDs) can be driven at currents from hundreds of mA to more than an ampere, compared with the tens of mA for other LEDs. Some can emit over a thousand lumens. LED power densities up to 300 W/cm² have been achieved. Since overheating is destructive, the HP-LEDs must be mounted on a heat sink to allow for heat dissipation. If the heat from an HP-LED is not removed, the device fails in seconds. One HP-LED can often replace an incandescent bulb in a flashlight, or be set in an array to form a powerful LED lamp.

Some well-known HP-LEDs in this category are the Nichia 19 series, Lumileds Rebel Led, Osram Opto Semiconductors Golden Dragon, and Cree X-lamp. As of September 2009, some HP-LEDs manufactured by Cree now exceed 105 lm/W.

Examples for Haitz's law—which predicts an exponential rise in light output and efficacy of LEDs over time—are the CREE XP-G series LED, which achieved 105 lm/W in 2009 and the Nichia 19 series with a typical efficacy of 140 lm/W, released in 2010.

AC driven

LEDs developed by Seoul Semiconductor can operate on AC power without a DC converter. For each half-cycle, part of the LED emits light and part is dark, and this is reversed during the next half-cycle. The efficacy of this type of HP-LED is typically 40 lm/W. A large number of LED elements in series may be able to operate directly from line voltage. In 2009, Seoul Semiconductor released a high DC voltage LED, named as 'Acrich MJT', capable of being driven from AC power with a simple controlling circuit. The low-power dissipation of these LEDs affords them more flexibility than the original AC LED design.

Application-specific variations

Flashing

Flashing LEDs are used as attention seeking indicators without requiring external electronics. Flashing LEDs resemble standard LEDs but they contain an integrated multivibrator circuit that causes the LED to flash with a typical period of one second. In diffused lens LEDs, this circuit is visible as a small black dot. Most flashing LEDs emit light of one color, but more sophisticated devices can flash between multiple colors and even fade through a color sequence using RGB color mixing.

Bi-color

Bi-color LEDs contain two different LED emitters in one case. There are two types of these. One type consists of two dies connected to the same two leads antiparallel to each other. Current flow in one direction emits one color, and current in the opposite direction emits the other color. The other type consists of two dies with separate leads for both dies and another lead for common anode or cathode so that they can be controlled independently. The most common bi-color combination is red/traditional green, however, other available combinations include amber/traditional green, red/pure green, red/blue, and blue/pure green.

Tri-color

Tri-color LEDs contain three different LED emitters in one case. Each emitter is connected to a separate lead so they can be controlled independently. A four-lead arrangement is typical with one common lead (anode or cathode) and an additional lead for each color.

RGB

RGB LEDs are tri-color LEDs with red, green, and blue emitters, in general using a four-wire connection with one common lead (anode or cathode). These LEDs can have either common positive or common negative leads. Others, however, have only two leads (positive and negative) and have a built-in tiny electronic control unit.

Decorative-multicolor

Decorative-multicolor LEDs incorporate several emitters of different colors supplied by only two lead-out wires. Colors are switched internally by varying the supply voltage.

Alphanumeric

Alphanumeric LEDs are available in seven-segment, starburst, and dot-matrix format. Seven-segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters. Dot-matrix displays typically use 5x7 pixels per character. Seven-segment LED displays were in widespread use in the 1970s and 1980s, but rising use of liquid crystal displays, with their lower power needs and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED displays.

Digital-RGB

Digital-RGB LEDs are RGB LEDs that contain their own "smart" control electronics. In addition to power and ground, these provide connections for data-in, data-out, and sometimes a clock or strobe signal. These are connected in a daisy chain, with the data in of the first LED sourced by a microprocessor, which can control the brightness and color of each LED independently of the others. They are used where a combination of maximum control and minimum visible electronics are needed such as strings for Christmas and LED matrices. Some even have refresh rates in the kHz range, allowing for basic video applications.

Filament

An LED filament consists of multiple LED chips connected in series on a common longitudinal substrate that forms a thin rod reminiscent of a traditional incandescent filament. These are being used as a low-cost decorative alternative for traditional light bulbs that are being phased out in many countries. The filaments require a rather high voltage to light to nominal brightness, allowing them to work efficiently and simply with mains voltages. Often a simple rectifier and capacitive current limiting are employed to create a low-cost replacement for a traditional light bulb without the complexity of the low voltage, high current converter that single die LEDs need. Usually, they are packaged in a sealed enclosure with a shape similar to lamps they were designed to replace (e.g. a bulb) and filled with inert nitrogen or carbon dioxide gas to remove heat efficiently.

Considerations for use

Power sources

Main article: LED power sources

Simple LED circuit with resistor for current limiting

The current–voltage characteristic of an LED is similar to other diodes, in that the current is dependent exponentially on the voltage (see Shockley diode equation). This means that a small change in voltage can cause a large change in current. If the applied voltage exceeds the LED's forward voltage drop by a small amount, the current rating may be exceeded by a large amount, potentially damaging or destroying the LED. The typical solution is to use constant-current power supplies to keep the current below the LED's maximum current rating. Since most common power sources (batteries, mains) are constant-voltage sources, most LED fixtures must include a power converter, at least a current-limiting resistor. However, the high resistance of three-volt coin cells combined with the high differential resistance of nitride-based LEDs makes it possible to power such an LED from such a coin cell without an external resistor.

Electrical polarity

Main article: Electrical polarity of LEDs

As with all diodes, current flows easily from p-type to n-type material. However, no current flows and no light is emitted if a small voltage is applied in the reverse direction. If the reverse voltage grows large enough to exceed the breakdown voltage, a large current flows and the LED may be damaged. If the reverse current is sufficiently limited to avoid damage, the reverse-conducting LED is a useful noise diode.

Safety and health

The vast majority of devices containing LEDs are "safe under all conditions of normal use", and so are classified as "Class 1 LED product"/"LED Klasse 1". At present, only a few LEDs—extremely bright LEDs that also have a tightly focused viewing angle of 8° or less—could, in theory, cause temporary blindness, and so are classified as "Class 2". The opinion of the French Agency for Food, Environmental and Occupational Health & Safety (ANSES) of 2010, on the health issues concerning LEDs, suggested banning public use of lamps in the moderate Risk Group 2, especially those with a high blue component, in places frequented by children.

In general, laser safety regulations—and the "Class 1", "Class 2", etc. system—also apply to LEDs.

While LEDs have the advantage over fluorescent lamps that they do not contain mercury, they may contain other hazardous metals such as lead and arsenic. Regarding the toxicity of LEDs when treated as waste, a study published in 2011 stated: "According to federal standards, LEDs are not hazardous except for low-intensity red LEDs, which leached Pb [lead] at levels exceeding regulatory limits (186 mg/L; regulatory limit: 5). However, according to California regulations, excessive levels of copper (up to 3892 mg/kg; limit: 2500), lead (up to 8103 mg/kg; limit: 1000), nickel (up to 4797 mg/kg; limit: 2000), or silver (up to 721 mg/kg; limit: 500) render all except low-intensity yellow LEDs hazardous."

In 2016 a statement of the American Medical Association (AMA) concerning the possible influence of blueish street lighting on the sleep-wake cycle of city-dwellers led to some controversy. So far high-pressure sodium lamps (HPS) with an orange light spectrum were the most efficient light sources commonly used in street-lighting. Now many modern street lamps are equipped with Indium gallium nitride LEDs (InGaN). These are even more efficient and mostly emit blue-rich light with a higher correlated color temperature (CCT). Since light with a high CCT resembles daylight it is thought that this might have an effect on the normal circadian physiology by suppressing melatonin production in the human body. There have been no relevant studies as yet and critics claim exposure levels are not high enough to have a noticeable effect.

Advantages

• Efficiency: LEDs emit more lumens per watt than incandescent light bulbs. The efficiency of LED lighting fixtures is not affected by shape and size, unlike fluorescent light bulbs or tubes.
• Color: LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.
• Size: LEDs can be very small (smaller than 2 mm²) and are easily attached to printed circuit boards.
• Warmup time: LEDs light up very quickly. A typical red indicator LED achieves full brightness in under a microsecond. LEDs used in communications devices can have even faster response times.
• Cycling: LEDs are ideal for uses subject to frequent on-off cycling, unlike incandescent and fluorescent lamps that fail faster when cycled often, or high-intensity discharge lamps (HID lamps) that require a long time before restarting.
• Dimming: LEDs can very easily be dimmed either by pulse-width modulation or lowering the forward current. This pulse-width modulation is why LED lights, particularly headlights on cars, when viewed on camera or by some people, appear to be flashing or flickering. This is a type of stroboscopic effect.
• Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
• Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.
• Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer. Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000 to 2,000 hours. Several DOE demonstrations have shown that reduced maintenance costs from this extended lifetime, rather than energy savings, is the primary factor in determining the payback period for an LED product.
• Shock resistance: LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile.
• Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner. For larger LED packages total internal reflection (TIR) lenses are often used to the same effect. However, when large quantities of light are needed many light sources are usually deployed, which are difficult to focus or collimate towards the same target.

Disadvantages

• Initial price: LEDs are currently slightly more expensive (price per lumen) on an initial capital cost basis, than other lighting technologies. As of March 2014, at least one manufacturer claims to have reached $1 per kilolumen. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed.
• Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment – or thermal management properties. Overdriving an LED in high ambient temperatures may result in overheating the LED package, eventually leading to device failure. An adequate heat sink is needed to maintain long life. This is especially important in automotive, medical, and military uses where devices must operate over a wide range of temperatures, which require low failure rates. Toshiba has produced LEDs with an operating temperature range of −40 to 100 °C, which suits the LEDs for both indoor and outdoor use in applications such as lamps, ceiling lighting, street lights, and floodlights.
• Voltage sensitivity: LEDs must be supplied with a voltage above their threshold voltage and a current below their rating. Current and lifetime change greatly with a small change in applied voltage. They thus require a current-regulated supply (usually just a series resistor for indicator LEDs).
• Color rendition: Most cool-white LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects to be perceived differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism, red surfaces being rendered particularly poorly by typical phosphor-based cool-white LEDs.
• Area light source: Single LEDs do not approximate a point source of light giving a spherical light distribution, but rather a lambertian distribution. So LEDs are difficult to apply to uses needing a spherical light field; however, different fields of light can be manipulated by the application of different optics or "lenses". LEDs cannot provide divergence below a few degrees. In contrast, lasers can emit beams with divergences of 0.2 degrees or less.
• Electrical polarity: Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity, LEDs only light with correct electrical polarity. To automatically match source polarity to LED devices, rectifiers can be used.
• Blue hazard: There is a concern that blue LEDs and cool-white LEDs are now capable of exceeding safe limits of the so-called blue-light hazard as defined in eye safety specifications such as ANSI/IESNA RP-27.1–05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems.
• Light pollution: Because white LEDs, especially those with high color temperature, emit much more short wavelength light than conventional outdoor light sources such as high-pressure sodium vapor lamps, the increased blue and green sensitivity of scotopic vision means that white LEDs used in outdoor lighting cause substantially more sky glow. The American Medical Association warned on the use of high blue content white LEDs in street lighting, due to their higher impact on human health and environment, compared to low blue content light sources (e.g. High-Pressure Sodium, PC amber LEDs, and low CCT LEDs).
• Efficiency droop: The efficiency of LEDs decreases as the electric current increases. Heating also increases with higher currents, which compromises LED lifetime. These effects put practical limits on the current through an LED in high power applications.
• Impact on insects: LEDs are much more attractive to insects than sodium-vapor lights, so much so that there has been speculative concern about the possibility of disruption to food webs.
• Use in winter conditions: Since they do not give off much heat in comparison to incandescent lights, LED lights used for traffic control can have snow obscuring them, leading to accidents.

Applications

LED uses fall into four major categories:

• Visual signals where light goes more or less directly from the source to the human eye, to convey a message or meaning
• Illumination where light is reflected from objects to give visual response of these objects
• Measuring and interacting with processes involving no human vision
• Narrow band light sensors where LEDs operate in a reverse-bias mode and respond to incident light, instead of emitting light

Indicators and signs

The low energy consumption, low maintenance and small size of LEDs has led to uses as status indicators and displays on a variety of equipment and installations. Large-area LED displays are used as stadium displays, dynamic decorative displays, and dynamic message signs on freeways. Thin, lightweight message displays are used at airports and railway stations, and as destination displays for trains, buses, trams, and ferries.

Red and green LED traffic signals

One-color light is well suited for traffic lights and signals, exit signs, emergency vehicle lighting, ships' navigation lights or lanterns (chromacity and luminance standards being set under the Convention on the International Regulations for Preventing Collisions at Sea 1972, Annex I and the CIE) and LED-based Christmas lights. In cold climates, LED traffic lights may remain snow-covered. Red or yellow LEDs are used in indicator and alphanumeric displays in environments where night vision must be retained: aircraft cockpits, submarine and ship bridges, astronomy observatories, and in the field, e.g. night time animal watching and military field use.

Automotive applications for LEDs continue to grow.

Because of their long life, fast switching times, and visibility in broad daylight due to their high output and focus, LEDs have been used in brake lights for cars' high-mounted brake lights, trucks, and buses, and in turn signals for some time. However, many vehicles now use LEDs for their rear light clusters. The use in brakes improves safety, due to a great reduction in the time needed to light fully, or faster rise time, up to 0.5 second faster than an incandescent bulb. This gives drivers behind more time to react. In a dual intensity circuit (rear markers and brakes) if the LEDs are not pulsed at a fast enough frequency, they can create a phantom array, where ghost images of the LED appear if the eyes quickly scan across the array. White LED headlamps are beginning to appear. Using LEDs has styling advantages because LEDs can form much thinner lights than incandescent lamps with parabolic reflectors.

Due to the relative cheapness of low output LEDs, they are also used in many temporary uses such as glowsticks, throwies, and the photonic textile Lumalive. Artists have also used LEDs for LED art.

Weather and all-hazards radio receivers with Specific Area Message Encoding (SAME) have three LEDs: red for warnings, orange for watches, and yellow for advisories and statements whenever issued.

Lighting

With the development of high-efficiency and high-power LEDs, it has become possible to use LEDs in lighting and illumination. To encourage the shift to LED lamps and other high-efficiency lighting, the US Department of Energy has created the L Prize competition. The Philips Lighting North America LED bulb won the first competition on August 3, 2011, after successfully completing 18 months of intensive field, lab, and product testing.

LEDs are used as street lights and in other architectural lighting. The mechanical robustness and long lifetime are used in automotive lighting on cars, motorcycles, and bicycle lights. LED light emission may be efficiently controlled by using nonimaging optics principles.

LED street lights are employed on poles and in parking garages. In 2007, the Italian village of Torraca was the first place to convert its entire illumination system to LEDs.

LEDs are used in aviation lighting. Airbus has used LED lighting in its Airbus A320 Enhanced since 2007, and Boeing uses LED lighting in the 787. LEDs are also being used now in airport and heliport lighting. LED airport fixtures currently include medium-intensity runway lights, runway centerline lights, taxiway centerline and edge lights, guidance signs, and obstruction lighting.

LEDs are also used as a light source for DLP projectors, and to backlight LCD televisions (referred to as LED TVs) and laptop displays. RGB LEDs raise the color gamut by as much as 45%. Screens for TV and computer displays can be made thinner using LEDs for backlighting.

The lack of IR or heat radiation makes LEDs ideal for stage lights using banks of RGB LEDs that can easily change color and decrease heating from traditional stage lighting, as well as medical lighting where IR-radiation can be harmful. In energy conservation, the lower heat output of LEDs also means air conditioning (cooling) systems have less heat in need of disposal.

LEDs are small, durable and need little power, so they are used in handheld devices such as flashlights. LED strobe lights or camera flashes operate at a safe, low voltage, instead of the 250+ volts commonly found in xenon flashlamp-based lighting. This is especially useful in cameras on mobile phones, where space is at a premium and bulky voltage-raising circuitry is undesirable.

LEDs are used for infrared illumination in night vision uses including security cameras. A ring of LEDs around a video camera, aimed forward into a retroreflective background, allows chroma keying in video productions.

LED for miners, to increase visibility inside mines

LEDs are used in mining operations, as cap lamps to provide light for miners. Research has been done to improve LEDs for mining, to reduce glare and to increase illumination, reducing risk of injury to the miners.

LEDs are now used commonly in all market areas from commercial to home use: standard lighting, AV, stage, theatrical, architectural, and public installations, and wherever artificial light is used.

LEDs are increasingly finding uses in medical and educational applications, for example as mood enhancement, and new technologies such as AmBX, exploiting LED versatility. NASA has even sponsored research for the use of LEDs to promote health for astronauts.

Data communication and other signalling

See also: Li-Fi

Light can be used to transmit data and analog signals. For example, lighting white LEDs can be used in systems assisting people to navigate in closed spaces while searching necessary rooms or objects.

Assistive listening devices in many theaters and similar spaces use arrays of infrared LEDs to send sound to listeners' receivers. Light-emitting diodes (as well as semiconductor lasers) are used to send data over many types of fiber optic cable, from digital audio over TOSLINK cables to the very high bandwidth fiber links that form the Internet backbone. For some time, computers were commonly equipped with IrDA interfaces, which allowed them to send and receive data to nearby machines via infrared.

Because LEDs can cycle on and off millions of times per second, very high data bandwidth can be achieved.

Sustainable lighting

Efficient lighting is needed for sustainable architecture. In 2009, US Department of Energy testing results on LED lamps showed an average efficacy of 35 lm/W, below that of typical CFLs, and as low as 9 lm/W, worse than standard incandescent bulbs. A typical 13-watt LED lamp emitted 450 to 650 lumens, which is equivalent to a standard 40-watt incandescent bulb.

However, as of 2011, there are LED bulbs available as efficient as 150 lm/W and even inexpensive low-end models typically exceed 50 lm/W, so that a 6-watt LED could achieve the same results as a standard 40-watt incandescent bulb. The latter has an expected lifespan of 1,000 hours, whereas an LED can continue to operate with reduced efficiency for more than 50,000 hours.

See the chart below for a comparison of common light types:

	LED	CFL	Incandescent
Lightbulb Projected Lifespan	50,000 hours	10,000 hours	1,200 hours
Watts Per Bulb (equiv. 60 watts)	10	14	60
Cost Per Bulb	$2.00	$7.00	$1.25
kWh of Electricity Used Over 50,000 Hours	500	700	3000
Cost of Electricity (@ 0.10 per kWh)	$50	$70	$300
Bulbs Needed for 50,000 Hours of Use	1	5	42
Equivalent 50,000 Hours Bulb Expense	$2.00	$35.00	$52.50
TOTAL Cost for 50,000 Hours	$52.00	$105.00	$352.50

Energy consumption

In the US, one kilowatt-hour (3.6 MJ) of electricity currently causes an average 1.34 pounds (610 g) of CO
2 emission. Assuming the average light bulb is on for 10 hours a day, a 40-watt bulb causes 196 pounds (89 kg) of CO
2 emission per year. The 6-watt LED equivalent only causes 30 pounds (14 kg) of CO
2 over the same time span. A building’s carbon footprint from lighting can, therefore, be reduced by 85% by exchanging all incandescent bulbs for new LEDs—if a building previously used only incandescent bulbs.

In practice, most buildings that use a lot of lighting use fluorescent lighting, which has 22% luminous efficiency compared with 5% for filaments, so changing to LED lighting would still give a 34% reduction in electrical power use and carbon emissions.

The reduction in carbon emissions depends on the source of electricity. Nuclear power in the United States produced 19.2% of electricity in 2011, so reducing electricity consumption in the U.S. reduces carbon emissions more than in France (75% nuclear electricity) or Norway (almost entirely hydroelectric).

Replacing lights that spend the most time lit results in the most savings, so LED lights in infrequently used locations bring a smaller return on investment.

Light sources for machine vision systems

Machine vision systems often require bright and homogeneous illumination, so features of interest are easier to process. LEDs are often used for this purpose, and this is likely to remain one of their major uses until the price drops low enough to make signaling and illumination uses more widespread. Barcode scanners are the most common example of machine vision, and many low-cost products use red LEDs instead of lasers. Optical computer mice are an example of LEDs in machine vision, as it is used to provide an even light source on the surface for the miniature camera within the mouse. LEDs constitute a nearly ideal light source for machine vision systems for several reasons:

• The size of the illuminated field is usually comparatively small and machine vision systems are often quite expensive, so the cost of the light source is usually a minor concern. However, it might not be easy to replace a broken light source placed within complex machinery, and here the long service life of LEDs is a benefit.
• LED elements tend to be small and can be placed with high density over flat or even-shaped substrates (PCBs etc.) so that bright and homogeneous sources that direct light from tightly controlled directions on inspected parts can be designed. This can often be obtained with small, low-cost lenses and diffusers, helping to achieve high light densities with control over lighting levels and homogeneity. LED sources can be shaped in several configurations (spot lights for reflective illumination; ring lights for coaxial illumination; backlights for contour illumination; linear assemblies; flat, large format panels; dome sources for diffused, omnidirectional illumination).
• LEDs can be easily strobed (in the microsecond range and below) and synchronized with imaging. High-power LEDs are available allowing well-lit images even with very short light pulses. This is often used to obtain crisp and sharp "still" images of quickly moving parts.
• LEDs come in several different colors and wavelengths, allowing easy use of the best color for each need, where different color may provide better visibility of features of interest. Having a precisely known spectrum lets tightly matched filters be used to separate informative bandwidth or to reduce disturbing effects of ambient light. LEDs usually operate at comparatively low working temperatures, simplifying heat management, and dissipation. This allows using plastic lenses, filters, and diffusers. Waterproof units can also easily be designed, allowing use in harsh or wet environments (food, beverage, oil industries).

• A large LED display behind a disc jockey

• LED digital display that can display four digits and points

• Traffic light using LED

• LED daytime running lights of Audi A4

• LED panel light source used in an experiment on plant growth. The findings of such experiments may be used to grow food in space on long duration missions.

• LED lights reacting dynamically to video feed via AmBX

• Different sized LEDs. 8 mm, 5 mm and 3 mm, with a wooden match-stick for scale.

• A green surface-mount colored LED mounted on an Arduino circuit board

Other applications

LED costume for stage performers

LED wallpaper by Meystyle

The light from LEDs can be modulated very quickly so they are used extensively in optical fiber and free space optics communications. This includes remote controls, such as for TVs, VCRs, and LED Computers, where infrared LEDs are often used. Opto-isolators use an LED combined with a photodiode or phototransistor to provide a signal path with electrical isolation between two circuits. This is especially useful in medical equipment where the signals from a low-voltage sensor circuit (usually battery-powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a recording or monitoring device operating at potentially dangerous voltages. An optoisolator also lets information be transferred between circuits that don't share a common ground potential.

Many sensor systems rely on light as the signal source. LEDs are often ideal as a light source due to the requirements of the sensors. LEDs are used as motion sensors, for example in optical computer mice. The Nintendo Wii's sensor bar uses infrared LEDs. Pulse oximeters use them for measuring oxygen saturation. Some flatbed scanners use arrays of RGB LEDs rather than the typical cold-cathode fluorescent lamp as the light source. Having independent control of three illuminated colors allows the scanner to calibrate itself for more accurate color balance, and there is no need for warm-up. Further, its sensors only need be monochromatic, since at any one time the page being scanned is only lit by one color of light. Since LEDs can also be used as photodiodes, they can be used for both photo emission and detection. This could be used, for example, in a touchscreen that registers reflected light from a finger or stylus. Many materials and biological systems are sensitive to, or dependent on, light. Grow lights use LEDs to increase photosynthesis in plants, and bacteria and viruses can be removed from water and other substances using UV LEDs for sterilization.

LEDs have also been used as a medium-quality voltage reference in electronic circuits. The forward voltage drop (e.g. about 1.7 V for a normal red LED) can be used instead of a Zener diode in low-voltage regulators. Red LEDs have the flattest I/V curve above the knee. Nitride-based LEDs have a fairly steep I/V curve and are useless for this purpose. Although LED forward voltage is far more current-dependent than a Zener diode, Zener diodes with breakdown voltages below 3 V are not widely available.

The progressive miniaturization of low-voltage lighting technology, such as LEDs and OLEDs, suitable to incorporate into low-thickness materials has fostered experimentation in combining light sources and wall covering surfaces for interior walls. The new possibilities offered by these developments have prompted some designers and companies, such as Meystyle, Ingo Maurer, Lomox and Philips, to research and develop proprietary LED wallpaper technologies, some of which are currently available for commercial purchase. Other solutions mainly exist as prototypes or are in the process of being further refined.

See also

• History of display technology
• Laser diode
• LEAD (diode), a light-emitting and absorbing diode
• LED circuit
• LED lamp
• LED tattoo
• Li-Fi
• Light-emitting electrochemical cell
• List of LED failure modes
• Nixie tube
• OLED
• Photovoltaics
• Seven-segment display
• SMD LED Module
• Solar lamp
• Solid-state lighting
• Thermal management of high-power LEDs
• UV curing

Further reading

• Shuji Nakamura; Gerhard Fasol; Stephen J Pearton (2000). The Blue Laser Diode: The Complete Story. Springer Verlag. ISBN 3-540-66505-6. 

External links

• Light-emitting diode at DMOZ
• Educational video on LEDs on YouTube

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English Journal

• Vaccines against Clostridium perfringens alpha-toxin.

• Nagahama M.Author information Department of Microbiology, Faculty of Pharmaceutical Sciences, Tokushima Bunri University, Yamashiro- cho, Tokushima 770-8514, Japan. nagahama@ph.bunri-u.ac.jp.AbstractClostridium perfringens alpha-toxin is thought to be an important agent in gas gangrene, which is a lifethreatening infection with fever, pain, edema, myonecrosis, and gas production. The toxin (370 residues) is composed of an N-terminal domain (1-250 residues, N-domain) in which the catalytic site is found and a C-terminal domain (251-370 residues, C-domain) responsible for binding to membranes. During the past decade, recombinant DNA technology has been employed to develop second-generation vaccines, including site-directed mutants and the C-domain of the toxin, to prevent gas gangrene. These immunities have led to protection against the lethal effects of wild-type C. perfringens in mice. C-domain vaccines are capable of protecting against heterologous clostridia causing clostridial myonecrosis. This article summarizes the current knowledge on vaccines against alpha-toxin.
• Current pharmaceutical biotechnology.Curr Pharm Biotechnol.2014 Nov;14(10):913-7.
• Clostridium perfringens alpha-toxin is thought to be an important agent in gas gangrene, which is a lifethreatening infection with fever, pain, edema, myonecrosis, and gas production. The toxin (370 residues) is composed of an N-terminal domain (1-250 residues, N-domain) in which the catalytic site
• PMID 24372250

• Evidence and uptake routes for Zinc oxide nanoparticles through the gastrointestinal barrier in Xenopus laevis.

• Bacchetta R1, Moschini E, Santo N, Fascio U, Del Giacco L, Freddi S, Camatini M, Mantecca P.Author information 1Department of Biosciences, Università degli Studi di Milano , Milano , Italy.AbstractThe developmental toxicity of nanostructured materials, as well as their impact on the biological barriers, represents a crucial aspect to be assessed in a nanosafety policy framework. Nanosized metal oxides have been demonstrated to affect Xenopus laevis embryonic development, with nZnO specifically targeting the digestive system. To study the mechanisms of the nZnO-induced intestinal lesions, we tested two different nominally sized ZnO nanoparticles (NPs) at effective concentrations. Advanced microscopy techniques and molecular marker analyses were applied in order to describe the NP-epithelial cell interactions and the mechanisms driving NP toxicity and translocation through the intestinal barrier. We attributed the toxicity to NP-induced cell oxidative damage, the small-sized NPs being the more effective. This outcome is sustained by a marked increase in anti-oxidant genes' expression and high lipid peroxidation level in the enterocytes, where disarrangement of the cytoskeleton and cell junctions' integrity were evidenced. These events led to diffuse necrotic changes in the intestinal barrier, and trans- and paracellular NP permeation through the mucosa. The uptake routes, leading NPs to cross the intestinal barrier and reach secondary target tissues, have been documented. nZnOs embryotoxicity was confirmed to be crucially mediated by the NPs' reactivity rather than their dissolved ions. The ZnO NPs' ability to overwhelm the intestinal barrier must be taken into high consideration for a future design of safer ZnO NPs.
• Nanotoxicology.Nanotoxicology.2014 Nov;8(7):728-44. doi: 10.3109/17435390.2013.824128. Epub 2013 Aug 6.
• The developmental toxicity of nanostructured materials, as well as their impact on the biological barriers, represents a crucial aspect to be assessed in a nanosafety policy framework. Nanosized metal oxides have been demonstrated to affect Xenopus laevis embryonic development, with nZnO specificall
• PMID 23848496

• Comparison between taxane-based chemotherapy with conventional surgery-based therapy for cutaneous angiosarcoma: a single-center experience.

• Fujisawa Y1, Nakamura Y, Kawachi Y, Otsuka F.Author information 1Department of Dermatology, University of Tsukuba , 1-1-1 Tennodai, Tsukuba , Japan.AbstractOBJECTIVE: To show the efficacy of taxane-based chemotherapy for the treatment of cutaneous angiosarcoma.
• The Journal of dermatological treatment.J Dermatolog Treat.2014 Oct;25(5):419-23. doi: 10.3109/09546634.2012.754839. Epub 2013 Jun 6.
• OBJECTIVE: To show the efficacy of taxane-based chemotherapy for the treatment of cutaneous angiosarcoma.METHODS: A case-control study comparing patients who received taxanes without wide local excision (group A, n = 5) and patients who received conventional surgery-based therapy (group B, n = 8) in
• PMID 23210914

Japanese Journal

• LEDを用いたアリーナのスポーツ照明と,その設置例

• 広島工業大学紀要. 研究編 51, 231-241, 2017-02
• NAID 40021085198

• 照明技術開発賞受賞 : GaNパワーデバイスを搭載した調光器対応電球形LEDランプ用小形点灯装置

• 照明学会誌 101(2), 89-92, 2017-02
• NAID 40021075609

• 次世代に科学を伝える映像心理大作戦(第2回)ノーベル賞受賞記念!青色LED,あまり知られていない真の価値

• 金属 87(2), 134-138, 2017-02
• NAID 40021072330

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