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Svjetleća dioda (LED), koja emitira svjetlost kada se napaja električno i stvara elektroluminiscenciju

Apr 21, 2017

Svjetleća dioda

Dioda koja emitira svjetlo
RBG-LED.jpg Plave, zelene i crvene LED diode u difuznom kućištu od 5 mm
Princip rada elektroluminiscencija
izumio H._J._Round (1907) [1]
Oleg Losev (1927) [2]
James R. Biard (1961) [3]
Nick Holonyak (1962) [4]
Prva produkcija Listopad 1962. godine
Konfiguracija kontakta Anoda i katoda
Elektronički simbol
LED symbol.svg


Dijelovi konvencionalne LED diode. Ravne dno površine nakovnja i postova ugrađene unutar epoksida djeluju kao sidra, kako bi se spriječilo prisilno izvlačenje vodiča mehaničkim naprezanjem ili vibracijama.











Moderni LED raspored s E27 vijkom u bazi


Svjetiljka s modernim retrofitnim LED lampom u obliku žarulje s aluminijskim hladnjakom , svjetlosnom difuzornom kupolom i vijčanim vijkom E27 , koristeći ugrađeni izvor napajanja koji radi na mrežnom naponu




Zatvori sliku LED zasuna na površini





Svjetleća dioda ( LED ) je dvosmjerni izvor poluvodiča . To je p-n spojna dioda koja emitira svjetlost kad se aktivira. [5] Kada se prikladni napon primijeni na vodi, elektroni se mogu rekombinirati s rupicama elektrona unutar uređaja, oslobađajući energiju u obliku fotona . Taj se učinak naziva elektroluminiscencija , a boja svjetlosti (koja odgovara energiji fotona) određena je jazom energetskog pojasa poluvodiča. LED su obično male (manje od 1 mm2) i integrirane optičke komponente mogu se koristiti za oblikovanje uzorka zračenja . [6]

Pojavivši se kao praktične elektroničke komponente 1962. godine, najranije LED diode emitiraju infracrvenu svjetlost malog intenziteta. Infracrvene LED diode se i dalje često koriste kao elementi za odašiljanje u daljinsko upravljanje krugovima, poput onih u daljinskim upravljačima za široku paletu potrošačke elektronike. Prve svjetleće svjetlosti s vidljivim svjetlom također su bile niskog intenziteta i ograničene na crveno. Moderne LED diode dostupne su preko vidljivih , ultraljubičastih i infracrvenih valnih duljina, uz vrlo visoku svjetlinu.

Rano LED diode često su se koristile kao indikatorske svjetiljke za elektroničke uređaje, zamjenjujući male žarulje sa žarnom niti. Uskoro su bili zapakirani u numeričke očitanja u obliku sedam segmentnih zaslona i obično su se vidjeli u digitalnim satovima. Nedavni razvoj LED dioda omogućuje im da se koriste u ekološkim i zadatnim osvjetljenjem. LED-ovi su dopustili razvoj novih zaslona i senzora, a njihovi visoki stupnjevi prijenosa također se koriste u naprednoj komunikacijskoj tehnologiji.

LED-ovi imaju mnogo prednosti u odnosu na izvore svjetlosti sa žarnom niti, uključujući nižu potrošnju energije, duži vijek trajanja, poboljšanu fizičku robusnost, manju veličinu i bržu izmjenu. Svjetleće diode se sada koriste u raznim primjenama kao što su zrakoplovna rasvjeta , automobilska prednja svjetla , oglašavanje, opća rasvjeta , prometni signali , bljeskalice fotoaparata i osvijetljena pozadina. Od 2017. godine, LED rasvjetna svjetla u kući su jeftina ili jeftinija od kompaktnih izvora fluorescentnih svjetiljki usporedivih rezultata. [8] Oni su također značajno energetski učinkoviti i, vjerojatno, imaju manje ekoloških problema povezanih s njihovim odlaganjem. [9] [10]


Sadržaj

[ Sakrij ]


Povijest [ uredi ]

Otkrića i raniji uređaji [ uredi ]

Zelena elektroluminescencija s točke kontakta na kristalu SiC rekreira Roundov izvorni eksperiment iz 1907.

Elektroluminescencija kao fenomen otkrila je 1907. britanski eksperimentator HJ Round of Marconi Labs , koristeći kristal silicijevog karbida i detektora mačjeg zuba . [11] [12] Ruski izumitelj Oleg Losev izvijestio je o stvaranju prve LED svjetiljke 1927. godine. [13] Njegovo je istraživanje distribuirano u sovjetskim, njemačkim i britanskim znanstvenim časopisima, ali više od desetljeća nije došlo do praktičnog korištenja tog otkrića. [ 15] Kurt Lehovec , Carl Accardo i Edward Jamgochian objasnili su ove prve svjetlosne diode 1951. godine pomoću uređaja koji koriste SiC kristale s trenutnim izvorom baterije ili generatora impulsa i usporedbom s varijantom čistog kristala U 1953. [16] [17]

Rubin Braunstein [18] Radio Corporation of America izvijestio je o infracrvenoj emisiji iz galalnog arsenida (GaAs) i ostalih poluvodičkih legura 1955. [19] Braunstein je opazio infracrvenu emisiju koju su stvorile jednostavne diodne strukture pomoću galijskog antimonida (GaSb), GaAs, indij Fosfid (InP) i silikonski-germanium (SiGe) legure na sobnoj temperaturi i na 77 Kelvin.

Godine 1957. Braunstein je nadalje pokazao da se rudimentarni uređaji mogu koristiti za ne-radio komunikaciju na maloj udaljenosti. Kao što je napomenuo Kroemer [20] Braunstein "... postavio je jednostavnu optičku komunikacijsku vezu: Glazba koja se pojavila s rekordera koristila se putem prikladne elektronike za moduliranje naprijedne struje GaAs-diode. Emitirana svjetlost otkrila je neka PbS dioda Ovaj signal je uložen u audio pojačalo i reproduciran od strane zvučnika, presretanje grede zaustavilo je glazbu, a imali smo veliku zabavu igrati s ovom postava. Ovo je postavljanje predvidalo upotrebu LED dioda za optičke komunikacijske aplikacije.

LED Instruments SNX-100 GaAs LED koji se nalazi u kućištu metalnog tranzistora TO-18.

U rujnu 1961., dok je radio u Texas Instrumentsu u Dallasu , Teksas , James R. Biard i Gary Pittman otkrili su svjetlosnu zraku blizu infra crvene (900 nm) iz tunelske diode koju su izgradili na GaAs supstratu. [7] Do listopada 1961. pokazali su učinkovitu emisiju svjetlosti i signalnu spojku između GaAs pn svjetlosnog odašiljača svjetla i električno izoliranog poluvodičkog fotodetektora. [8] 8. kolovoza 1962. godine, Biard i Pittman podnijeli su patent pod nazivom "Semiconductor Radiant Diode" na temelju njihovih nalaza, koji su opisali cinkov difuzni p-n spojni LED s razmaknutim katodnim kontaktom kako bi se omogućila učinkovita emisija infracrvene svjetlosti pod Naprijed pristranost . Nakon utvrđivanja prioriteta njihovog rada na temelju inženjerskih bilježnica prije predavanja GE Labs, RCA Research Labs, IBM Research Labs, Bell Labs i Lincoln Lab na MIT-u , američki patentni ured izdao je dva izumitelja patenta za infracrvene GaAs (IR ) Svjetlosne diode (US Patent US3293513 ), prva praktična LED dioda. [7] Odmah nakon podnošenja patenta, Texas Instruments (TI) započeo je projekt proizvodnje infracrvenih dioda. U listopadu 1962. TI je najavio prvi komercijalni LED proizvod (SNX-100) koji je koristio čisti kristalni GaAs koji emitira svjetlosni izlaz od 890 nm. [7] U listopadu 1963. TI je najavio prvi komercijalni polusferni LED, SNX-110. [22]

Prva LED svjetla s vidljivim spektrom (crvena) razvila je 1962. godine Nick Holonyak, Jr. dok je radio u Generalu Electric . Holonyak je prvi put objavio svoj LED u časopisu Applied Physics Letters 1. prosinca 1962. [22] [24] M. George Craford , bivši diplomski student Holonyaka, izumio je prvu žutu LED i poboljšala svjetlinu crvene i Crveno-narančaste LED-a za faktor deset u 1972. [26] 1976. godine TP Pearsall je stvorio prve LED diode velike svjetlosti i visoke učinkovitosti za telekomunikacije optičkih vlakana izumom novih poluvodičkih materijala posebno prilagođenih valnim duljinama prijenosa optičkih vlakana. [27]

Početni komercijalni razvoj [ uredi ]

Prve komercijalne LED diode obično su korištene kao zamjena za žarulje sa žarnom niti i neonskom svjetiljkom i na sedam segmentnih zaslona , [28] prvo u skupe opreme kao što su laboratorijske i elektroničke testne opreme, a kasnije u takvim aparatima kao što su TV, radio, telefoni, Kalkulatora, kao i satova (vidi popis signala koristi ). Do 1968. vidljive i infracrvene LED diode bile su iznimno skupe, po redoslijedu od 200 USD po jedinici, pa su imale malo praktične upotrebe. [29] Tvrtka Monsanto bila je prva organizacija koja je 1968. godine proizvodila svjetleće LED diode pomoću galijevog arsenid fosfida (GaAsP) za proizvodnju crvenih LED dioda prikladnih za pokazatelje. [29] Tvrtka Hewlett Packard (HP) uvela je LED diode 1968. godine, u početku korištenja GaAsP-a koju je isporučio Monsanto. Ove crvene LED diode bile su dovoljno svijetle samo za upotrebu kao pokazatelji, budući da izlaz svjetla nije dovoljno da osvijetli prostor. Čitači u računalima bili su toliko mali da su plastične leće napravljene iznad svake znamenke kako bi bile čitljive. Kasnije su druge boje postale široko dostupne i pojavile se u aparatima i opremi. U 1970-ima komercijalno uspješni LED uređaji za manje od pet centa bili su proizvedeni od Fairchild Optoelectronics. Ti su uređaji koristili složene poluvodičke čipove proizvedene planarnim postupkom koji je izumio dr. Jean Hoerni u tvrtki Fairchild Semiconductor . [30] [31] Kombinacija planerske obrade za izradu čipova i inovativnih metoda pakiranja omogućila je timu u Fairchildu predvodio pionir optoelektronike Thomas Brandt kako bi se postigla potrebna smanjenja troškova. [32] Ove metode i dalje koriste LED proizvođači. [33]

LED zaslon TI-30 znanstvenog kalkulatora (oko 1978) koji koristi plastične leće za povećanje vidljive veličine znamenki

Većina LED dioda je izrađena u vrlo uobičajenim paketima od 5 mm T1¾ i 3 mm T1, no uz rastuće snage, sve je više potrebna kako bi se izbacila suvišna toplina radi održavanja pouzdanosti, [34] tako da su složenije pakete prilagođene za učinkovitu disperziju topline , Paketi za najsuvremenije visoke snage LED-ovi malo podsjećaju na rane LED diode.

Plava LED [ uredi ]

Plave LED diode prvi put su razvili Herbert Paul Maruska na RCA 1972. godine pomoću galij nitrida (GaN) na safirnom supstratu. [35] [36] SiC-vrste prvi su komercijalno prodavane u Sjedinjenim Državama od strane Cree 1989. [37] Međutim, niti jedna od ovih početnih plavih LED dioda nije bila jako svijetla.

Prva visoka svjetlost plava LED pokazala je Shuji Nakamura od Nichia Corporation 1994. godine, a temelji se na InGaN-u . [38] [39] Paralelno, Isamu Akasaki i Hiroshi Amano u Nagoji su radili na razvoju važne GaN nukleacije na safirnim supstratima i demonstraciji p-tipa dopiranja GaN-a. Nakamura, Akasaki i Amano dobili su Nobelovu nagradu za fiziku 2014. za svoj rad. [40] Godine 1995. Alberto Barbieri sa Sveučilišnog laboratorija u Cardiffu (GB) istražio je učinkovitost i pouzdanost LED svjetla visoke svjetline i pokazala LED "transparentni kontakt" pomoću indijskog kositraznog oksida (ITO) na (AlGaInP / GaAs).

Godine 2001. [41] i 2002. [42] uspješno su demonstrirani postupci za povećanje LED dialita galijeva nitrida (GaN) na siliciju . U siječnju 2012. Osram je demonstrirao velike snage InGaN LED-a koji su komercijalno rasli na silicijskim podlogama. [43]

Bijele LED diode i proboj osvjetljenja [ uredi ]

Postizanje visoke učinkovitosti u plavim LED zaslonom brzo prati razvoj prve bijele LED . U ovom uređaju Y
Al Al
5 O
12 : Ce (poznat kao " YAG ") fosforna prevlaka na emiteru upija neke plave emisije i proizvodi žuto svjetlo kroz fluorescenciju . Kombinacija te žute boje s preostalim plavim svjetlom pojavljuje se blijedo oko. Međutim, upotrebom raznih fosfora (fluorescentnih materijala) također je postalo moguće da umjesto toga proizvode zelenu i crvenu svjetlost kroz fluorescenciju. Dobivena mješavina crvene, zelene i plave ne samo da ih ljudi smatraju bijelim svjetlom već je superiorniji za osvjetljenje u smislu prikazivanja boje , dok se ne može cijeniti boja crvenih ili zelenih predmeta osvijetljenih jedino žutim (i preostalim plavim) Valnih duljina iz YAG fosfora.

Ilustracija Haitzovog zakona , pokazujući poboljšanje svjetlosnog izlaza po LED-u tijekom vremena, s logaritamskom mjerom na vertikalnoj osi

Prve bijele LED diode bile su skupo i neučinkovite. Međutim, svjetlosni izlaz LED-a povećao se eksponencijalno , a udvostručenje se događa približno svakih 36 mjeseci od šezdesetih godina prošlog stoljeća (slično Mooreovom zakonu ). Ovaj trend općenito se pripisuje paralelnom razvoju ostalih poluvodičkih tehnologija i napretku u optici [ citat potreban ] i znanosti o materijalima i nazvan je Haitzov zakon nakon Dr. Rolanda Haitz. [44]

Izlaz svjetlosti i učinkovitost plavih i bliskih ultraljubičastih LED dioda porasli su kao pada pouzdanih uređaja: to je dovelo do uporabe (relativno) LED dioda s bijelim svjetlima velike snage u svrhu osvjetljavanja koje zamjenjuju žarulju i fluorescentnu rasvjetu. [45] [46]

Pokazano je da eksperimentalne bijele LED diode proizvode više od 300 lumena po watu električne energije; Neki mogu trajati do 100.000 sati. [47] U usporedbi s žaruljama sa žarnom niti, ovo nije samo veliko povećanje električne učinkovitosti, ali - s vremenom - slična ili niža cijena po žarulji. [48]

Radno načelo [ uredi ]

Unutarnji rad LED-a, koji prikazuje krug (vrh) i dijagrama trake (dno)

PN spoj može pretvoriti apsorbiranu svjetlosnu energiju u proporcionalnu električnu struju. Isti postupak je obrnuto ovdje (tj. PN spajanje emitira svjetlost kad se na nju primijeni električna energija). Taj se fenomen općenito naziva elektroluminiscencija , koja se može definirati kao emisija svjetla iz poluvodiča pod utjecajem električnog polja . Nosači punjenja se rekombiniraju u naprijed usmjerenom PN spoju kada se elektroni kreću od N-regije i rekombiniraju s rupama koje postoje u P-regiji. Slobodni elektroni su u vodljivom nivou energetske razine, dok su rupe u valnom području . Stoga će razina energije otvora biti manja od energetskih razina elektrona. Neki dio energije mora se raspršiti kako bi se rekombinirao elektrone i rupe. Ta se energija emitira u obliku topline i svjetlosti.

Elektroni raspršuju energiju u obliku topline za silicij i germijenske diode, ali u galijskom arsenid fosfidu (GaAsP) i galij fosfidnim (GaP) poluvodičima, elektroni raspršuju energiju emitiranjem fotona . Ako je poluvodič proziran, prijelaz postaje izvor svjetlosti kako se emitira, tako da postaje svjetleća dioda, ali kada je spajanje preokrenuto obrnuto, LED neće proizvesti svjetlo, a ako je potencijal dovoljno velik, Uređaj će biti oštećen.

Tehnologija [ uredi ]

IV dijagram za diodu . LED će početi emitirati svjetlost kada se na njega primijeni više od 2 ili 3 volta. Okretna regija polariteta koristi različite vertikalne ljestvice od prednjeg područja polariteta, kako bi se pokazalo da je struja propuštanja gotovo konstantna s naponom sve dok ne dođe do kvara. U prednaponu napona, struja je malena, ali eksponencijalno raste s naponom.

Fizika [ uredi ]

LED se sastoji od čipa poluvodičkog materijala dopiranog s nečistoćama kako bi se stvorio pn spajanje . Kao iu drugim diodama, struja teče lako s p-strane, ili anode , na n-strani ili katoda, ali ne u obrnutom smjeru. Nosači naboja - elektroni i rupe - prelijevaju u spoj iz elektroda s različitim naponom. Kada elektron susreće rupu, pada u nižu razinu energije i oslobađa energiju u obliku fotona .

Valna duljina emitirane svjetlosti, a time i njezine boje, ovisi o energetskoj snazi materijala materijala koji tvore pn spoj . U silicijskim ili germijanskim diodama, elektroni i rupe obično se rekombiniraju ne-radijalnom tranzicijom , koja ne proizvodi nikakvu optičku emisiju, jer su to neizravni materijali za bend . Materijali koji se upotrebljavaju za LED imaju izravan presjek benda s energijama koje odgovaraju bliskom infracrvenom, vidljivom ili blizu ultraljubičastom svjetlu.

Razvoj LED-a započeo je infracrvenim i crvenim uređajima napravljenim s galijevim arsenidom . Napredak u znanosti o materijalima omogućio je stvaranje uređaja sa sve kraćim valnim duljinama, emitirajući svjetlo u različitim bojama.

LED-ovi se obično grade na supstratu n-tipa, s elektrodom pričvršćenom na sloj p-tipa smješten na svojoj površini. P-tip supstrata, iako manje uobičajeni, javljaju kao dobro. Mnogi komercijalni LEDs, posebno GaN / InGaN, također koriste sapphire supstrat.

Indeks refrakcije [ uredi ]

Idealizirani primjer čunjeva za emitiranje svjetlosti u jednostavnom kvadratnom poluvodiču, za jednu zonu emisije iz izvora točke. Lijevi ilustracija je za prozirnu maticu, dok je desna ilustracija prikazana polu-čunjaka formirana kada je donji sloj neproziran. Svjetlost emitira jednako u svim smjerovima od točke-izvora, ali može pobjeći samo okomito na površinu poluvodiča i na nekim stupnjevima na strani, što je ilustrirano pomoću oblika konusa. Kada se prekorači kritični kut, fotoni se reflektiraju interno. Područja između čunjeva predstavljaju zarobljenu svjetlosnu energiju izgubljenu kao vrućina. [49] Većina materijala koji se koriste za proizvodnju LED-a imaju vrlo visoke indekse refrakcije . To znači da će se većina svjetla reflektirati natrag u materijal na sučelju površine materijala / zraka. Dakle, vađenje svjetlosti u LED- ima važan je aspekt proizvodnje LED-a, podložno mnogo istraživanja i razvoja. Čajevi za emitiranje svjetlosti pravih LED svjetiljki daleko su složenija od one emisije svjetla iz točke. Zona emisije svjetlosti tipično je dvodimenzionalna ravnina između walera. Svaki atom preko ove ravnine ima pojedinačni skup emajnih čunjeva. Crtanje milijarda preklopnih čunjeva nemoguće je, stoga je to pojednostavljeni dijagram s prikazom opsega svih kombiniranih štapića. Veće bočne češere su urezane kako bi se prikazale unutrašnje značajke i smanjila složenost slike; Oni bi se protezali do suprotnih rubova dvodimenzionalne emisijske ravnine.

Goli neprevučeni poluvodiči kao što je silicij pokazuju vrlo visoku indeks loma u odnosu na otvoreni zrak, koji sprečava prolazak fotona koji dolaze pod oštrim kutovima u odnosu na površinu koja se kontaktira s zrakom poluvodiča zbog potpunog unutarnjeg refleksije . Ova svojstva utječu i na učinkovitost emitiranja svjetlosti LED-a, kao i na učinkovitost apsorpcije svjetlosti fotonaponskih ćelija . Indeks loma silikona iznosi 3.96 (pri 590 nm), [50] dok je zrak 1.0002926. [51]

Općenito, čelik s LED površinskim poluvodičkim česticama koji se neprerađuje, emitira svjetlo samo okomito na površinu poluvodiča, a nekoliko stupnjeva prema strani, u obliku konusa koji se naziva svjetlosnim konusom , konusom svjetlosti [52] ili bijega Konus . [49] Maksimalni kut incidencije naziva se kritički kut . Kad se prekorači ovaj kut, fotoni više ne bježe iz poluvodiča nego se interno reflektiraju unutar kristala poluvodiča kao da su zrcalo . [49]

Interni refleksi mogu pobjeći kroz druga kristalna lica ako je kut incidencije dovoljno nizak i kristal je dovoljno proziran da ne apsorbira emisiju fotona. No, za jednostavnu četvrtastu LED s 90 stupnjeva kosih površina sa svih strana, lica svi djeluju kao jednaki kutni zrcala. U ovom slučaju većina svjetla ne može pobjeći i izgubljena je kao otpadna toplina u kristalu. [49]

Zavarena površina čipova sa zakrivljenim stranama slična draguljima ili fresnelovom ležištu može povećati svjetlosni izlaz dopuštajući da se svjetlost emitira okomito na površinu čipa, dok je daleko na stranu fotonske točke emisije. [53]

Idealni oblik poluvodiča s maksimalnim izlaznim svjetlom bio bi mikrosfera s emisijom fotona koja se pojavljuje u točnom centru, s elektrodama koje prodiru do središta na kontakt na mjestu emisije. Sve svjetlosne zrake koje proizlaze iz središta bile bi okomite na cijelu površinu kugle, što nije rezultiralo unutarnjim refleksijama. Također bi radilo polukuglasti poluvodič, s ravnom stražnjom površinom koja služi kao zrcalo na natrag razbacane fotone. [54]

Prijelazne prevlake [ uredi ]

Nakon dopiranja napolitanog , odvaja se u pojedinačne umre . Svaki um je obično nazvan čipom.

Mnogi LED poluvodički čipovi su kapsulirani ili postavljeni u čistu ili obojenu kalupljenu plastičnu školjku. Plastična školjka ima tri svrhe:

  1. Ugradnja poluvodičkog čipa u uređaje je lakše ostvariti.

  2. Sićušna krhka električna ožičenja fizički je podržana i zaštićena od oštećenja.

  3. Plastika djeluje kao reflektirajući posrednik između relativno visokog indeksa poluvodiča i otvorenog zraka niske indekse. [55]

Treća značajka pomaže u povećanju emisije svjetlosti od poluvodiča djelovanjem kao difuzijska leća, čime se svjetlost emitira na mnogo višem kutu učestalosti od svjetlosnog konusa od onog koji je jedini čip moguće emitirati sam.

Učinkovitost i operativni parametri [ uredi ]

Tipične LED indikatore dizajnirane su za rad s ne više od 30-60 milliwata (mW) električne energije. Oko 1999, Philips Lumileds uveo LED napajanja sposobna za kontinuirano korištenje na jednom watta . Ove LED diode koriste mnogo veće poluvodičke veličine umora za rukovanje velikim ulazima snage. Također, poluvodički umovi postavljeni su na metalne čepove kako bi se omogućilo uklanjanje toplote iz LED dioda.

Jedna od ključnih prednosti LED rasvjetnih izvora je visoka svjetlosna učinkovitost . Bijele LED diode brzo su se prilagodile i dostigle učinkovitost standardnih žarulja. Godine 2002. Lumileds je na raspolaganju osvijetlio pet watt LED svjetiljki s svjetlosnom učinkovitošću od 18-22 lumena po watu (lm / W). Za usporedbu, konvencionalna žarulja sa žarnom niti od 60-100 W emitira oko 15 lm / W, a standardna fluorescentna svjetla emitiraju do 100 lm / W.

Od 2012, Philips je postigao sljedeće učinkovitosti za svaku boju. [56] Vrijednosti učinkovitosti pokazuju snagu fizike - svjetlost po električnoj energiji. Vrijednost djelotvornosti lumena po vatu uključuje značajke ljudskog oka i izvedena je pomoću funkcije svjetlosti .


Boja Raspon valnih duljina (nm) Tipični koeficijent učinkovitosti Tipična učinkovitost ( lm / W )

crvena 620 <>λ <> 0.39 72

Crveno-narančasta 610 <>λ <> 0.29 98

zelena 520 <>λ <> 0.15 93

cijan 490 <>λ <> 0.26 75

plava 460 <>λ <> 0.35 37

U rujnu 2003. godine Cree je pokazao novu vrstu plave LED svjetiljke koja troši 24 mW na 20 miliamperi (mA). To je proizvelo komercijalno zapakirano bijelo svjetlo koje je dala 65 lm / W pri 20 mA, postajući najsvjetlija bijela LED komercijalno dostupna u to vrijeme i više od četiri puta učinkovitija od standardnih incandansa. Godine 2006. demonstrirali su prototip s rekordnom bijelom LED svjetlosnom učinkovitošću od 131 lm / W pri 20 mA. Tvrtka Nichia Corporation razvila je bijelu LED svjetiljku s jakim djelotvornosti od 150 lm / W pri naprijedoj struji od 20 mA. [57] Creeovi XLamp XM-L LEDs, komercijalno dostupni u 2011, proizvode 100 lm / W uz punu snagu od 10 W i do 160 lm / W oko 2 W ulazne snage. U 2012. godini, Cree je objavio bijelu LED diodu koja je u ožujku 2014. godine iznosila 254 lm / W, [58] i 303 lm / W. [59] Praktična opća rasvjeta zahtijeva LED diode visoke snage od jednog watta ili više. Tipične radne struje za takve uređaje počinju na 350 mA.

Ove učinkovitosti su samo za svjetlosnu diodu koja se održava na niskoj temperaturi u laboratoriju. Budući da LED diode instalirane u stvarnim svjetiljkama rade na višoj temperaturi, a gubitci vozača, učinkovitost u stvarnom svijetu znatno je manja. Ispitivanje komercijalnih LED svjetiljki dizajniranih za zamjenu žarulja ili CFL-ova pokazalo je da je prosječna učinkovitost bila još oko 46 lm / W u 2009. (ispitani rezultati variraju od 17 lm / W do 79 lm / W). [60]

Učinkovitost droga [ uredi ]

Učinkovitost je smanjenje svjetlosne učinkovitosti LED-a jer se električna struja povećavaju iznad desetaka milliampera.

Ovaj efekt je u početku bio teoriziran da se odnosi na povišene temperature. Znanstvenici su se pokazali suprotnim da su istiniti: iako bi se život LED-a smanjio, smanjenje učinkovitosti je manje ozbiljno na povišenim temperaturama. [61] Mehanizam koji uzrokuje smanjenje djelotvornosti identificiran je 2007. godine kao Augerova rekombinacija , koja je uzeta miješanom reakcijom. [62] Godine 2013. studija je potvrdila rekombinaciju Augera kao uzroka smanjenja učinkovitosti. [63]

Pored toga što su manje učinkoviti, LED-ovi koji rade na višim električnim strujama stvaraju veću razinu topline što ugrožava životni vijek LED-a. Zbog toga povećanog zagrijavanja pri većim strujama, LED diode visoke svjetline imaju industrijski standard koji rade na samo 350 mA, što je kompromis između izlaza svjetlosti, učinkovitosti i dugovječnosti. [62] [64] [65] [66]

Moguća rješenja [ uredi ]

Umjesto povećanja trenutne razine, osvjetljenje se obično povećava kombiniranjem višestrukih LED dioda u jednoj žarulji. Rješavanje problema smanjenja učinkovitosti značilo bi da kućna LED žarulja trebaju manje LED dioda, što bi značajno smanjilo troškove.

Istraživači u US Naval Research Laboratory pronašli su način da se smanji učinkovitost droop. Otkrili su da droop proizlazi iz ne-radiativne Auger rekombinacije injektiranih nosača. Oni su stvorili kvantne jažice s mekim potencijalom za ograničavanje kako bi se smanjili ne-radijalni procesi Augera. [67]

Istraživači Tajvanskog nacionalnog sveučilišta i Epistar Corp razvijaju način smanjenja učinkovitosti droopova pomoću aluminijskih nitrida (AlN) koji su toplinski vodljiviji od komercijalno korištene safirske vode. Viša toplinska vodljivost smanjuje efekte samo-grijanja. [68]

Trajanje i neuspjeh [ uredi ]

Glavni članak: Popis LED modova kvara

Uređaji sa čvrstim stanjima, kao što su LED, podložni su vrlo ograničenom trošenju i istrošenosti ako rade pri niskim strujama i pri niskim temperaturama. Navedeni tipični vijek trajanja iznosi 25.000 do 100.000 sati, ali postavke topline i struje mogu znatno produljiti ili skratiti ovo vrijeme. [69]

Najčešći simptom pogreške LED (i diodnog lasera ) je postupno smanjenje svjetlosnog snopa i gubitak učinkovitosti. Iznenadni kvarovi, iako rijetki, također se mogu pojaviti. Rane crvene LED diode su bile značajne zbog kratkog vijeka trajanja. S razvojem LED dioda visoke snage, uređaji se podvrgavaju višim temperaturama spajanja i većim gustoćama struje od tradicionalnih uređaja. To uzrokuje stres na materijalu i može uzrokovati degradaciju rane svjetlosne emisije. Kako bi se kvantitativno klasificiralo korisno vijek trajanja na standardizirani način predloženo je korištenje L70 ili L50, koji su runtimes (obično dani u tisućama sati), na kojima određena LED doseže 70% i 50% početne svjetlosne snage. [70]

Budući da u većini prethodnih izvora svjetlosti (žarulje sa žarnom niti, raspršene svjetiljke i one koje spaljuju gorivo, npr. Svijeće i uljne svjetiljke) svjetlost proizlazi iz topline, LED-ovi rade samo ako su dovoljno hladni. Proizvođač obično određuje maksimalnu temperaturu spajanja od 125 ili 150 ° C, a niže temperature su poželjne u interesu dugog vijeka trajanja. Na tim temperaturama, zračenje gubi relativno malo topline, što znači da je svjetlosna zraka koju proizvodi LED dioda super.

Otpadna toplina u LED-u velike snage (koja od 2015. godine može biti manja od polovice snage koju troši) prenosi se provođenjem kroz podlogu i paket LED-a na hladnjak , koji daje toplinu ambijentu Zrak konvekcijom. Pažljivi termički dizajn stoga je neophodan, uzimajući u obzir toplinske otpornosti LED paketa, hladnjaka i sučelja između dva. LED diode srednje snage često su oblikovane tako da se izravno lemaju na ploču s tiskanom pločicom koja sadrži toplinski vodljivi metalni sloj. LEDs velike snage pakiraju se u velike površinske keramičke pakete namijenjene za pričvršćivanje na metalni hladnjak , a sučelje kao materijal s visokom toplinskom vodljivošću ( toplinska mast , materijal za promjenu faze , toplinski vodljivi sloj ili toplinski ljepilo ).

Ako je svjetiljka s LED zasunom ugrađena u neizloženu rasvjetu , ili se rasvjetno tijelo nalazi u okruženju bez slobodne cirkulacije zraka, LED će se vjerojatno pregrijati, što će rezultirati smanjenjem životnog vijeka ili ranijim katastrofalnim neuspjehom. Termičko oblikovanje često se temelji na temperaturi okoline od 25 ° C (77 ° F). LED-ovi koji se koriste u vanjskim aplikacijama, kao što su prometni signali ili signalna svjetla u vozilu, te u klimatskim uvjetima gdje temperatura unutar svjetlosnog učvršćenja postaje vrlo visoka, mogla bi doživjeti smanjenje izlaza ili čak neuspjeh. [71]

Budući da je LED učinkovitost veća pri niskim temperaturama, LED tehnologija je prikladna za rasvjetu zamrzivača u supermarketu. [72] [73] [74] Budući da LED proizvodi manje otpadne topline od žarulja sa žarnom niti, njihova uporaba u zamrzivaču može uštedjeti i troškove hlađenja. Međutim, oni mogu biti osjetljiviji na smrzavanje i nakupljanje snijega od žarulja sa žarnom niti, [71] tako da su neki LED sustavi osvjetljenja dizajnirani s dodanim krugom grijanja. Osim toga, istraživanja su razvila tehnologije toplinskog sudopera koji će prenijeti toplinu proizvedenu unutar spoja na odgovarajuća područja svjetiljke. [75]

Boje i materijali [ uredi ]

Konvencionalne LED diode izrađene su od različitih anorganskih poluvodičkih materijala . Sljedeća tablica prikazuje dostupne boje s rasponom valnih duljina, padom napona i materijalom:


Boja Valna duljina [nm] Pad napona [ΔV] Poluvodički materijal

Infracrveni Λ > 760 ΔV <> Gallium arsenid (GaAs)
Aluminij galijev arsenid (AlGaAs)

crvena 610 <>λ <> 1,63 <> Aluminij galijev arsenid (AlGaAs)
Gallium arsenid fosfid (GaAsP)
Aluminijski galijev indij fosfid (AlGaInP)
Galijev (III) fosfid (GaP)

narančasta 590 <>λ <> 2.03 <> Gallium arsenid fosfid (GaAsP)
Aluminijski galijev indij fosfid (AlGaInP)
Galijev (III) fosfid (GaP)

Žuta boja 570 <>λ <> 2.10 <> Gallium arsenid fosfid (GaAsP)
Aluminijski galijev indij fosfid (AlGaInP)
Galijev (III) fosfid (GaP)

zelena 500 <>λ <> 1.9 [76] <> Tradicionalno zeleno:
Galijev (III) fosfid (GaP)
Aluminijski galijev indij fosfid (AlGaInP)
Aluminijski galijev fosfid (AlGaP)
Čisti zeleni:
Indijalni galij nitrid (InGaN) / galijev (III) nitrid (GaN)

plava 450 <>λ <> 2.48 <> Cink selenid (ZnSe)
Indijalni galij nitrid (InGaN)
Silicij karbid (SiC) kao supstrat
Silicij (Si) kao supstrat - u razvoju

ljubičasta 400 <>λ <> 2.76 <> Indijalni galij nitrid (InGaN)

purpurna boja Višestruki tipovi 2.48 <> Dva plava / crvena LED dioda,
Plava s crvenom fosfornom,
Ili bijelo s ljubičastom plastikom

ultraljubičast Λ <> 3 <δv><> Indij galijev nitrid (InGaN) (385-400 nm)

Dijamant (235 nm) [77]
Boronski nitrid (215 nm) [78] [79]
Aluminium nitride (AlN) (210 nm) [80]
Aluminium gallium nitride (AlGaN)
Aluminium gallium indium nitride (AlGaInN)—down to 210 nm [81]


Pink Multiple types Δ V ~ 3.3 [82] 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. [83]


bijela Broad spectrum 2.8 < δ="">V <> Cool / Pure White: Blue/UV diode with yellow phosphor
Warm White: Blue diode with orange phosphor

Blue and ultraviolet [ edit ]

Blue LEDs

External video
Herb Maruska original blue LED College of New Jersey Sarnoff Collection.png
“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. [84] [85] 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. [86] [87] In 1974 the US Patent Office awarded Maruska, Rhines and Stanford professor David Stevenson a patent for their work in 1972 (US Patent US3819974 A ) and today magnesium-doping of gallium nitride continues to be the basis for all commercial blue LEDs and laser diodes. These devices built in the early 1970s had too little light output to be of 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). [88] SiC LEDs had very low efficiency, no more than about 0.03%, but did emit in the blue portion of the visible light spectrum. [ Citat potreban ]

In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping [89] 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. [90] 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. [91] 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). [ citation needed ] 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. [ Citat potreban ]

Nakamura was awarded the 2006 Millennium Technology Prize for his invention. [92] Nakamura, Hiroshi Amano and Isamu Akasaki were awarded the Nobel Prize in Physics in 2014 for the invention of the blue LED. [93] [94] [95] In 2015, a US court ruled that three companies (ie 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. [96]

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 will emit 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. [ Citat potreban ]

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. [97] 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. [98] UV-C wavelengths were obtained in laboratories using aluminium nitride (210 nm), [80] boron nitride (215 nm) [78] [79] and diamond (235 nm). [77]

RGB [ edit ]

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 [ edit ]

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 [99] —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) [100]

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 [ edit ]

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, [101] and in principle, this mechanism also has higher quantum efficiency in producing white light. [ Citat potreban ]

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 will mean 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, [102] 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. [103]

Phosphor-based LEDs [ edit ]

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 Ce 3+ :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). [104] 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. [105]

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 (Ce 3+ :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 [ edit ]

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. [106]

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. [107] 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. [108]

Organic light-emitting diodes (OLEDs) [ edit ]

Main article: Organic light-emitting diode

Demonstration of a flexible OLED device

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 . [109] The organic materials can be small organic molecules in a crystalline phase , or polymers . [110]

The potential advantages of OLEDs include thin, low-cost displays with a low driving voltage, wide viewing angle, and high contrast and color gamut. [111] Polymer LEDs have the added benefit of printable and flexible displays. [112] [113] [114] 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. [110] [111]

Quantum dot LEDs [ edit ]

See also: quantum dot display

Quantum dots (QD) are semiconductor nanocrystals whose optical properties allow their emission color to be tuned from the visible into the infrared spectrum. [115] [116] 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. [117]

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 . [118] Quantum dots are also being considered for use in white light-emitting diodes in liquid crystal display (LCD) televisions. [119]

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. [120] 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. [121]

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 . [122]

Types [ edit ]

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. [123]

Miniature [ edit ]

Photo of miniature surface mount LEDs in most common sizes. They can be much smaller than a traditional 5 mm lamp type LED which is 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 . [124] 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 . [125]

There are three main categories of miniature single die LEDs:

Low-current


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

Standard 20mA LEDs (ranging from approximately 40mW to 90mW) at around:
  • 1.9 to 2.1V for red, orange, yellow, and traditional green

  • 3.0 to 3.4V for pure green and blue

  • 2.9 to 4.2V for violet, pink, purple and white

Ultra-high-output


20mA at approximately 2 or 4–5V, designed for viewing in direct sunlight 5V and 12VLEDs are ordinary miniature LEDs that incorporate a suitable series   resistor for direct connection to a 5V or 12V supply.

High-power [ edit ]

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. [126] [127] LED power densities up to 300 W/cm 2 have been achieved. [128] 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 will fail 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. [129]

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 [129] and the Nichia 19 series with a typical efficacy of 140 lm/W, released in 2010. [130]

AC driven [ edit ]

LEDs have been developed by Seoul Semiconductor that can operate on AC power without the need for 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. [131] 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. [132]

Application-specific variations [ edit ]

Flashing [ edit ]

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 [ edit ]

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 [ edit ]

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 [ edit ]

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 [ edit ]

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 [ edit ]

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 [ edit ]

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 [ edit ]

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. [133] 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 creating a low voltage, high current converter which is required by single die LEDs. [134] Usually, they are packaged in a sealed enclosure with a shape similar to lamps they were designed to replace (eg a bulb) and filled with inert nitrogen or carbon dioxide gas to remove heat efficiently.

Considerations for use [ edit ]

Power sources [ edit ]

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. [135] 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 [ edit ]

Main article: Electrical polarity of LEDs

As with all diodes, current flows easily from p-type to n-type material. [136] 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 [ edit ]

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". [137] 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 which were in the moderate Risk Group 2, especially those with a high blue component in places frequented by children. [138] In general, laser safety regulations—and the "Class 1", "Class 2", etc. system—also apply to LEDs. [139]

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." [140]

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. [141]

Advantages [ edit ]

  • Efficiency: LEDs emit more lumens per watt than incandescent light bulbs. [142] 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 mm 2 [143] ) and are easily attached to printed circuit boards.

  • Warmup time: LEDs light up very quickly. A typical red indicator LED will achieve full brightness in under a microsecond . [144] 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. [145] 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. [69]

  • 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. [146] 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. [147]

  • 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 [ edit ]

  • 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. [148] 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. [107]

  • 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). [149]

  • 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 , [150] 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. [151]

  • Electrical polarity : Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity , LEDs will 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. [152] [153]

  • 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 . [132] [154] [155] [156] [157] 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 (eg High-Pressure Sodium, PC amber LEDs, and low CCT LEDs). [158]

  • Efficiency droop : The efficiency of LEDs decreases as the electric current increases. Heating also increases with higher currents which compromises the lifetime of the LED. These effects put practical limits on the current through an LED in high power applications. [62] [64] [65] [159]

  • 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. [160] [161]

  • 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. [162] [163]

Applications [ edit ]

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 [164]

  • Narrow band light sensors where LEDs operate in a reverse-bias mode and respond to incident light, instead of emitting light [165] [166] [167] [168]

Indicators and signs [ edit ]

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. [169] 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, eg night time animal watching and military field use.

Automotive applications for LEDs continue to grow.

Because of their long life, fast switching times, and their ability to be seen 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, but 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 [ citation needed ] 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 will appear if the eyes quickly scan across the array. White LED headlamps are starting to be used. 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 [ edit ]

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. [170]

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. [171]

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. [172]

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 to be used 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. [173]

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, [ citation needed ] and new technologies such as AmBX , exploiting LED versatility. NASA has even sponsored research for the use of LEDs to promote health for astronauts. [174]

Data communication and other signalling [ edit ]

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. [175]

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. [176]

Sustainable lighting [ edit ]

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, [177] 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 [ edit ]

In the US, one kilowatt-hour (3.6 MJ) of electricity currently causes an average 1.34 pounds (610 g) of CO
2
emission. [178] Assuming the average light bulb is on for 10 hours a day, a 40-watt bulb will cause 196 pounds (89 kg) of CO
2
emission per year. The 6-watt LED equivalent will only cause 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 US 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 [ edit ]

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. [179] 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 allows tightly matched filters to 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). [179]

Other applications [ edit ]

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 allows information to be transferred between circuits not sharing 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 . [180] Many materials and biological systems are sensitive to, or dependent on, light. Grow lights use LEDs to increase photosynthesis in plants , [181] and bacteria and viruses can be removed from water and other substances using UV LEDs for sterilization . [98]

LEDs have also been used as a medium-quality voltage reference in electronic circuits. The forward voltage drop (eg 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 be incorporated into low-thickness materials has fostered in recent years the experimentation on combining light sources and wall covering surfaces to be applied onto interior walls. [182] The new possibilities offered by these developments have prompted some designers and companies, such as Meystyle , [183] Ingo Maurer , [184] Lomox [185] and Philips , [186] 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.