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Svetleča dioda (LED), ki oddaja svetlobo pri električnem nabiranju elektroluminiscence

Apr 21, 2017

Svetleča dioda

Svetleča dioda
RBG-LED.jpg Modre, zelene in rdeče LED v 5 mm razpršenem ohišju
Delovno načelo Elektroluminiscenca
Izumljeno H._J._Round (1907) [1]
Oleg Losev (1927) [2]
James R. Biard (1961) [3]
Nick Holonyak (1962) [4]
Prva proizvodnja Oktober 1962
Konfiguracija pin Anoda in katoda
Elektronski simbol
LED simbol.svg


Deli običajne LED. Ploske spodnje površine nakovala in posta, vgrajene v epoksi del, kot sidra, da se prepreči, da se vodniki silovito izvlečejo s pomočjo mehanskih obremenitev ali vibracij.











Moderna LED naknadno opremljena z vijakom E27 v bazi


Zunanja svetilka v obliki žarnice z aluminijastim hladilnikom , svetlobno difuzijsko kupolo in vijačno podnožje E27 z vgrajenim napajalnikom, ki deluje na omrežno napetost




Zapri sliko LED za površinsko montažo





Svetleča dioda ( LED ) je dvolinijski polprevodniški svetlobni vir . To je p-n spojna dioda , ki oddaja svetlobo, ko je aktivirana. [5] Kadar se na vodnike nanese primerna napetost , se elektroni lahko rekombinirajo z elektronskimi luknjami v napravi in sproščajo energijo v obliki fotonov . Ta učinek se imenuje elektroluminiscenca , barva svetlobe (ki ustreza energiji fotona) pa se določi z razponom energijskih pasov polprevodnika. LED so običajno majhne (manj kot 1 mm 2 ) in integrirane optične komponente se lahko uporabljajo za oblikovanje vzorca sevanja . [6]

Leta 1962 so se pojavile kot praktične elektronske komponente [7], prve svetleče diode so oddale infrardečo svetlobo z nizko intenzivnostjo. Infrardeče LED se še vedno pogosto uporabljajo kot oddajni elementi v daljinskem upravljalnem krogu, kot so daljinski upravljalniki za široko paleto potrošniške elektronike. Prve svetleče diode z vidno svetlobo so bile tudi nizke intenzitete in omejene na rdeče. Moderne LED diode so na voljo preko vidnih , ultravijoličnih in infrardečih valovnih dolžin z zelo visoko svetlostjo.

Zgodnje LED so bile pogosto uporabljene kot indikatorske luči za elektronske naprave, ki so zamenjale majhne žarnice z žarilno nitko. Kmalu so bili pakirani v numerične odčitke v obliki sedem segmentnih prikazov in so bili običajno vidni v digitalnih urah. Nedavni razvoj LED-ov omogoča, da se jih uporablja pri osvetlitvi okolja in nalog. Svetleče diode so omogočile razvoj novih zaslonov in senzorjev, medtem ko se njihove visoke stopnje preklapljanja uporabljajo tudi v napredni komunikacijski tehnologiji.

LED imajo številne prednosti pred žariščnimi svetlobnimi viri, vključno z manjšo porabo energije, daljšo življenjsko dobo, izboljšano fizično robustnostjo, manjšo velikostjo in hitrejšim preklopom. Svetleče diode se zdaj uporabljajo v različnih aplikacijah, kot so letalska razsvetljava , avtomobilski žarometi , oglaševanje, splošna razsvetljava , prometni signali , utripa kamere in osvetljena ozadja. Od leta 2017, LED luči doma osvetlitev sobe so kot poceni ali cenejši od kompaktnih fluorescentnih svetilke virov primerljive proizvodnje. [8] Prav tako so bistveno bolj energetsko učinkovite in imajo verjetno manj okoljskih vprašanj, povezanih z njihovim odstranjevanjem. [9] [10]


Vsebina

[ Skrij ]


Zgodovina [ uredi ]

Odkritja in zgodnje naprave [ uredi ]

Zelena elektroluminiscenca iz točkovnega kontakta na kristalu SiC ponovno vzpostavi Rundev prvotni poskus od leta 1907.

Electroluminescence kot pojav je leta 1907 odkril britanski eksperimentator HJ Round of Marconi Labs , ki je uporabljal kristal silicijevega karbida in detektorja mačka . [11] [12] Ruski izumitelj Oleg Losev je poročal o ustanovitvi prve svetle diode leta 1927. [13] Njegova raziskava je bila razdeljena v sovjetskih, nemških in britanskih znanstvenih revijah, vendar praktično ni bila odkrita več desetletij. [ 15] Kurt Lehovec , Carl Accardo in Edward Jamgochian sta leta 1951 razložili prve svetleče diode z uporabo aparata, ki uporablja kristale SiC s trenutnim izvorom baterijskega ali pulznega generatorja in primerjavo z različico, čistim, kristalom Leta 1953. [16] [17]

Rubin Braunstein [18] iz Radio Corporation of America je leta 1955 poročal o infrardečih emisijah iz galijevega arzenida (GaAs) in drugih polprevodniških zlitin. [19] Braunstein je opazil infrardeče emisije, ki jih ustvarjajo preproste diode s pomočjo galijevega antimonida (GaSb), GaAs, indija Fosfidne (InP) in zlitine silicija-germanija (SiGe) pri sobni temperaturi in pri 77 Kelvinih.

Leta 1957 je Braunstein dokazal, da se lahko primarne naprave uporabljajo za ne-radijske komunikacije na kratki razdalji. Kakor je ugotovil Kroemer [20], je Braunstein "... vzpostavil preprosto optično komunikacijsko povezavo: Glasba, ki se je začela s predvajalnim predvajalnikom, je bila uporabljena z ustrezno elektroniko za moduliranje prednjega toka diode GaAs. Emitirano svetlobo je zaznala dioda PbS Ta signal je bil vnesen v avdio ojačevalnik in ga je predvajal zvočnik. Prezračevanje snopa je ustavilo glasbo. Imeli smo veliko zabave igrati s to nastavitvijo. " Ta nastavitev je predstavljala uporabo LED za optične komunikacijske aplikacije.

LED dioda Texas Instruments SNX-100 GaAs, vsebovana v TO-18 tranzistorski kovinski ohišji.

Septembra 1961, medtem ko je delal pri Texas Instruments v Dallasu v Teksasu , sta James R. Biard in Gary Pittman odkrila svetlobno emisijo blizu infrardeče (900 nm) iz tunelske diode, ki so jo zgradili na substratu GaAs. [7] Do oktobra 1961 so dokazali učinkovito oddajanje svetlobe in signalno povezavo med svetlobnim oddajnikom pN junction in Gauss električnim izoliranim polprevodnikovim fotodetektorjem. [21] 8. avgusta 1962 sta Biard in Pittman na podlagi svojih ugotovitev vložila patent z naslovom "Semiconductor Radiant Diode", ki je opisala LED diode p-n, ki so bile razporejene na cink, z razmaknjenim katodnim kontaktom, kar omogoča učinkovito izhajanje infrardeče svetlobe pod Napredno pristranskost . Po določitvi prednostne naloge svojega dela, ki temelji na inženirskih prenosnikih, preden so predložili pripombe podjetja GE Labs, RCA Research Labs, IBM Research Labs, Bell Labs in Lincoln Lab v MIT , je patentni urad ZDA izdal dva izumitelja patent za infrardečo povezavo GaAs ) Svetlečo diodo (US patent US3293513 ), prva praktična LED. [7] Takoj po vložitvi patenta je Texas Instruments (TI) začel projekt izdelave infrardečih diod. Oktobra 1962 je TI objavil prvi komercialni LED izdelek (SNX-100), ki je uporabljal čisti kristal GaAs, da oddaja 890 nm svetlobnih izhodov. [7] Oktobra 1963 je TI objavil prvo komercialno hemisferično LED, SNX-110. [22]

Prvi vidni spekter (rdeča) LED je razvil leta 1962 Nick Holonyak, Jr., medtem ko je delal na General Electric . Holonyak je najprej poročal o LED v reviji Applied Physics Letters 1. decembra 1962. [23] [24] M. George Craford , [25] nekdanji diplomant Holonyka, je izumil prvo rumeno LED in izboljšal svetlost rdečih in Rdeče-oranžne svetleče diode s faktorjem deset leta 1972. [26] Leta 1976 je TP Pearsall ustvaril prve visoke svetlosti, visoko učinkovitost LED za telekomunikacije z optičnimi vlakni z izumljanjem novih polprevodniških materialov, ki so posebej prilagojeni valovni dolžini optičnih vlaken. [27]

Začetni komercialni razvoj [ uredi ]

Prve komercialne LED so bile pogosto uporabljene kot zamenjava za žarnice z žarilno nitko in neonsko svetilko ter v sedmih segmentnih prikazih [28], najprej v dragi opremi, kot so laboratorijska in elektronska oprema za testiranje, pozneje pa v napravah, kot so televizije, radii, telefoni, Kalkulatorji, kot tudi ure (glej seznam signalov ). Do leta 1968 so bile vidne in infrardeče svetleče diode izjemno drage po 200 USD na enoto in tako imele malo praktične uporabe. [29] Podjetje Monsanto je bila prva organizacija za množično proizvodnjo vidnih LED, ki je leta 1968 uporabljala galijev arzenid fosfid (GaAsP) za proizvodnjo rdečih LED, primernih za indikatorje. [29] Hewlett Packard (HP) je leta 1968 uvedel LED diode, pri čemer je najprej uporabljal GaAsP, ki ga je dobavil Monsanto. Te rdeče svetleče diode so bile dovolj svetle le za uporabo kot indikatorji, saj svetloba ni bila dovolj za osvetlitev območja. Izpisi v kalkulatorjih so bili tako majhni, da so bile plastične leče zgrajene nad vsako številko, da so bile berljive. Kasneje so ostale barve postale splošno dostopne in se pojavile v napravah in opremi. V sedemdesetih letih so komercialno uspešne LED naprave pri manj kot petih centih proizvajale Fairchild Optoelectronics. Te naprave so uporabile sestavljene polprevodniške čipe, izdelane s ploskovnim postopkom, ki ga je izumil dr. Jean Hoerni pri Fairchild Semiconductor . [30] [31] Kombinacija planarne obdelave za izdelavo čipov in inovativne metode pakiranja je omogočila ekipo v Fairchildu, ki jo je pod vodstvom optoelektronike pionir Thomas Brandt dosegel potrebno znižanje stroškov. [32] Te metode še naprej uporabljajo proizvajalci LED. [33]

LED prikaz znanstvenega kalkulatorja TI-30 (približno 1978), ki uporablja plastične leče za povečanje vidne velikosti

Večina svetlečih diod je bila izdelana v zelo pogostih 5 mm T1¾ in 3 mm T1 paketih, vendar je z naraščajočo močjo vedno bolj potrebno, da se ohrani presežna toplota, da se ohrani zanesljivost [34], zato so bolj zapleteni paketi prilagojeni za učinkovito odvajanje toplote . Paketi za najsodobnejše visoko zmogljive LED diode so malo podobni zgodnjim LED.

Modra LED [ uredi ]

Modre LED je prvič razvil Herbert Paul Maruska pri RCA leta 1972 z uporabo galijevega nitrida (GaN) na osnovi safirja. [35] [36] Vrste SiC so najprej komercialno prodali v Združenih državah Amerike leta 1989. [37] Vendar nobena od teh začetnih modrih LED-jev ni bila zelo svetla.

Prvo visoko svetlo modro LED je pokazala Shuji Nakamura iz Nichia Corporation leta 1994 in je temeljila na InGaN . [38] [39] Hkrati so Isamu Akasaki in Hiroshi Amano v Nagoyi delali na razvoju pomembne GaN nukleacije na safirnih substratih in dokazovanju p-tipa dopinga GaN. Nakamura, Akasaki in Amano so prejeli Nobelovo nagrado za fiziko leta 2014 za svoje delo. [40] Leta 1995 je Alberto Barbieri v laboratoriju Univerze v Cardiffu (GB) raziskal učinkovitost in zanesljivost LED svetil z visoko svetlostjo in pokazal LED z "transparentnim kontaktom" z indijskim kositrovim oksidom (ITO) na (AlGaInP / GaAs).

Leta 2001 [41] in 2002, [42] so bili uspešno dokazani postopki za povečanje LED dioda galijevega nitrida (GaN) na silicij . Januarja 2012 je Osram demonstriral visoke moči InGaN LED, ki se tržijo na silicijah. [43]

Bele diode in osvetlitev preboj [ uredi ]

Za doseganje visoke učinkovitosti pri modrih LED-jih je hitro sledilo razvoj prve bele LED . V tej napravi je Y
3 Al
5 O
12 : Ce (poznan kot " YAG ") fosforni premaz na oddajniku absorbira nekaj modrega emisij in proizvaja rumeno svetlobo s fluorescenco . Kombinacija tega rumenega in preostale modre svetlobe se zdi belo za oko. Vendar pa je z uporabo različnih fosforjev (fluorescenčnih materialov) tudi s fluorescenco postalo mogoče ustvariti zeleno in rdečo svetlobo. Nastala mešanica rdečega, zelenega in modrega ne zaznava le ljudje kot bela svetloba, temveč je boljša za osvetlitev z vidika barvnega odrezovanja , medtem ko ne moremo ceniti barve rdečih ali zelenih predmetov, ki jih osvetljuje le rumena (in preostala modra) Valovne dolžine iz YAG fosforja.

Ilustracija Haitzovega zakona , ki kaže na izboljšanje svetlobe na svetlečo diodo skozi čas, z logaritmično lestvico na navpični osi

Prve bele LED so bile drage in neučinkovite. Vendar pa se je svetlobni izhod LED povečal eksponentno , podvojitev pa se je pojavila približno vsakih 36 mesecev od šestdesetih let 20. stoletja (podobno kot Moorejev zakon ). Ta trend je ponavadi pripisan vzporednemu razvoju drugih polprevodniških tehnologij ter napredku na področju optike in znanosti o materialih in se imenuje Haitzov zakon po doktorju Rolandu Haitzu. [44]

Svetlobni učinki in učinkovitost modrih in skoraj ultravijoličnih LED sta se povečala, ko so se stroški zanesljivih naprav zmanjšali: to je privedlo do uporabe (relativno) visoko močnih LED belih lučk za namene osvetlitve, ki nadomeščajo žarnico in fluorescenčno razsvetljavo. [45] [46]

Preizkušene bele LED so bile dokazane, da proizvedejo več kot 300 lumnov na vat električnega toka; Nekateri lahko trajajo do 100.000 ur. [47] V primerjavi z žarnicami z žarilno nitko je to ne samo ogromno povečanje električne učinkovitosti, ampak - sčasoma - podoben ali nižji strošek na žarnico. [48]

Delovno načelo [ uredi ]

Notranje delovanje LED, ki prikazuje vezje (vrh) in pasovni diagram (spodaj)

PN spoj lahko pretvori absorpcijsko energijo svetlobe v proporcionalni električni tok. Isti postopek se tukaj obrne (tj. Križišče PN oddaja svetlobo, ko se nanjo uporablja električna energija). Ta pojav se običajno imenuje elektroluminiscenca , ki jo lahko definiramo kot emisijo svetlobe iz polprevodnika pod vplivom električnega polja . Nosilci polnjenja rekombinirajo v vnaprej pristranskem PN-križišču kot elektroni, ki prečkajo N-regijo in rekombinirajo z luknjami, ki obstajajo v P-regiji. Prosti elektroni so v območju prevodnosti energetskih ravni, medtem ko so luknje v valenčni energijski pasovi . Tako bo raven energije lukenj manjša od ravni energije elektrona. Nekateri del energije je treba razpršiti, da bi rekombinirali elektrone in luknje. Ta energija se sprošča v obliki toplote in svetlobe.

Elektroni razpršijo energijo v obliki toplote za silicijeve in germanijeve diode, ampak v polprevodnikih galijevega arzenida fosfida (GaAsP) in galijevega fosfida (GaP), elektrone razpršijo energijo z emitiranjem fotonov . Če je polprevodnik prosojni, križišče postane vir svetlobe, ko se oddaja, s čimer postane svetleča dioda, toda kadar je križišče vzvratno pristransko, svetloba ne bo proizvedla LED, in če je potencial dovolj velik, Naprava bo poškodovana.

Tehnologija [ uredi ]

IV diagram za diode . LED bo začel oddajati svetlobo, ko se nanjo uporabi več kot 2 ali 3 volti. Območje povratne pristranskosti uporablja drugačno navpično lestvico iz prednje prednastavljene regije, da bi pokazalo, da je uhajanje toka skoraj konstantno z napetostjo, dokler ne pride do okvare. V naprej pristranskosti je tok majhen, vendar povečuje eksponentno z napetostjo.

Fizika [ uredi ]

LED je sestavljen iz čipa polprevodniškega materiala, dopiranega z nečistočami, da bi ustvarili pn križišče . Kot pri drugih diodah, tok zlahka poteka iz p-strani ali anode na n-stran ali katodo, vendar ne v obratni smeri. Nosilci polnjenja - elektroni in luknje - prehajajo v križišče iz elektrod z različnimi napetostmi. Ko elektron doseže luknjo, pade na nižjo raven energije in sprošča energijo v obliki fotona .

Valovna dolžina oddajane svetlobe in s tem njena barva je odvisna od energije pasovne vrzeli materialov, ki tvorijo pn križišče . V silikonskih ali germanijevih diodah se elektroni in luknje ponavadi rekombinirajo z ne-sevalnim prehodom , ki ne proizvaja optičnih emisij, ker gre za indirektne tračne materiale. Materiali, uporabljeni za LED, imajo direktno pasovno vrzel z energijami, ki ustrezajo skoraj infrardeči, vidni ali skoraj ultravijolični svetlobi.

Razvoj LED se je začel z infrardečimi in rdečimi napravami, izdelanimi z galijevim arzenidom . Napredek v znanosti o materialih je omogočil izdelavo naprav z vedno krajšimi valovnimi dolžinami, ki oddajajo svetlobo v različnih barvah.

LED so ponavadi zgrajene na n-tipu substrata, z elektrodo, pritrjeno na plast p-type, ki je nanosana na njeno površino. Podobne vrste P, čeprav so manj pogoste, se pojavljajo tudi. Veliko komercialnih LED, še posebej GaN / InGaN, uporablja tudi safirno podlago.

Indeks refrakcije [ uredi ]

Idealiziran primer stožcev emisij svetlobe v preprostem kvadratnem polprevodniku za eno točkovno območje emisij. Leva ilustracija je za prosojni rezin, medtem ko na desni ilustraciji prikažejo polovice stožcev, oblikovane, ko je spodnji sloj neprozoren. Svetloba se dejansko oddaja v vseh smereh iz točkovnega vira, vendar se lahko izogne pravokotno na površino polprevodnika in nekaj stopinj na stran, kar ponazarjajo oblike stožca. Ko je kritični kot prekoračen, se fotoni odražajo znotraj. Območja med stožci predstavljajo ujeto svetlobo, ki se porabi kot toplota. [49] Večina materialov, ki se uporabljajo za proizvodnjo LED, imajo zelo visoke refraktivne indekse . To pomeni, da se bo večina svetlobe odrazila nazaj v material na vmesniku materiala / zraka. Tako je lahka ekstrakcija v LED -ju pomemben vidik proizvodnje LED, odvisno od velikega števila raziskav in razvoja. Lahki oddajniki v realnem LED-rezervoarju so veliko bolj zapleteni kot enojna svetlobna emisija iz točke. Območje emisij svetlobe je običajno dvodimenzionalna ravnina med rezinami. Vsak atom na tej ravnini ima posamezen niz emisijskih stožcev. Risba milijard preklopnih stožcev je nemogoča, zato je to poenostavljen diagram, ki prikazuje razsežnosti vseh emisijskih stožcev skupaj. Večji stranski stožci so obrnjeni, da kažejo notranje značilnosti in zmanjšajo zapletenost slike; Bi se razširili na nasprotne robove dvodimenzionalne emisijske ploskve.

Goli neprevlečeni polprevodniki, kot je silicij, imajo zelo visok refrakcijski indeks glede na odprt zrak, kar preprečuje prehod fotonov, ki prihajajo do ostrih kotov glede na površino polprevodnika, ki se dotika zraka, zaradi celotnega notranjega refleksija . Ta lastnost vpliva tako na učinkovitost svetlobnih emisij LED kot tudi na svetlobno absorpcijsko učinkovitost fotonapetostnih celic . Lomni količnik silicija je 3,96 (pri 590 nm), [50] medtem ko je zrak 1.0002926. [51]

Na splošno bo polprevodniški čip z nepremazanimi polprevodniškimi čipi na svetlobi oddal svetlobo le pravokotno na površino polprevodnika in nekaj stopinj na stran, v obliki stožca, ki se imenuje svetlobni stožec , stožec svetlobe [52] ali izhod Stožec . [49] Največji kot naključnosti se imenuje kritični kot . Ko je ta kot prekoračen, fotoni ne pobegnejo več iz polprevodnika, ampak se namesto tega znotraj notranjosti polprevodniškega kristala kot ogledalo . [49]

Notranje refleksije lahko uidejo skozi druge kristalne obraze, če je naklonski kot dovolj nizek in kristal je dovolj prozoren, da ne absorbira fotonske emisije. Ampak za preprosto kvadratno LED z 90-stopinjskimi kotnimi površinami na vseh straneh, obrazi delujejo kot enako kotno ogledalo. V tem primeru se večina svetlobe ne more izogniti in se izgubi kot odpadna toplota v kristalu. [49]

Zgibana površina čipa s poševnimi fasetami, podobnimi dragulj ali fresnelovim lečam, lahko poveča svetlobni učinek tako, da se svetloba oddaja pravokotno na površino čipa, daleč stran, na straneh izstopne točke fotona. [53]

Idealna oblika polprevodnika z največjo svetlobno močjo bi bila mikrosfera z emisijo fotona v točno določenem središču, pri čemer bi elektrode prodrle v središče in se dotaknile na mestu izpusta. Vsi svetlobni žarki, ki izvirajo iz centra, bi bili pravokotni na celotno površino krogle, kar ne bi imelo nobenih notranjih odsevov. Prav tako bi delal hemisferični polprevodnik, pri čemer bi ravna hrbtna površina služila kot zrcalo za povratno raztresene fotone. [54]

Prehodni premazi [ uredi ]

Po dopingu vafla se razreže na posamezne matrice . Vsaka mrtva se običajno imenuje čip.

Številni LED-polprevodniški čipi so vdelani v embalažo ali v loncu v čistih ali obarvanih plastičnih lupinah. Plastična lupina ima tri namene:

  1. Namestitev polprevodniškega čipa v naprave je lažje doseči.

  2. Majhna krhka električna napeljava je fizično podprta in zaščitena pred poškodbami.

  3. Plastika deluje kot refrakcijski posrednik med relativno visokim indeksom polprevodnikov in nizkim indeksom odprtega zraka. [55]

Tretja funkcija pripomore k povečanju emisije svetlobe iz polprevodnika z delujočim difuzijskim lečo, ki omogoča oddajanje svetlobe s precej višjim kotom pojavljanja iz svetlobnega stožca, kot je le golo čip sposoben emitira sam.

Učinkovitosti in operativni parametri [ uredi ]

Tipične LED indikatorji so zasnovani tako, da delujejo z največjo močjo 30-60 mW (mW) električne energije. Okoli leta 1999 je Philips Lumileds uvedel LED diode, ki se lahko stalno uporabljajo pri enem vatu . Te svetleče diode so uporabile veliko večje polprevodniške dimenzije za obdelavo velikih močnostnih vložkov. Tudi polprevodniške matrice so bile nameščene na kovinske pločevinke, da bi omogočile odstranjevanje toplote iz LED matrice.

Ena od ključnih prednosti svetlobnih virov na osnovi LED je visoka svetlobna učinkovitost . Bela LED dioda se hitro ujemala in prehitela učinkovitost standardnih žarilnih sistemov. V letu 2002 so Lumileds izdelali LED diode s petimi vati s svetlobno učinkovitostjo 18-22 lumnov na vat (lm / W). Za primerjavo običajna žarnica z žarilno nitko od 60 do 100 vatov oddaja približno 15 lm / W, standardne fluorescentne svetilke pa oddajajo do 100 lm / W.

Od leta 2012 je Philips dosegel naslednje učinkovitosti za vsako barvo. [56] Vrednosti učinkovitosti prikazujejo fiziko - moč svetlobe na električno energijo. Vrednost učinkovitosti lumena na uro vključuje značilnosti človeškega očesa in izhaja iz funkcije svetilnosti .


Barva Območje valovnih dolžin (nm) Tipičen koeficient izkoristka Tipična učinkovitost ( lm / W )

rdeča 620 <>λ <> 0,39 72

Rdeče-oranžna 610 <>λ <> 0,29 98

Zelena 520 <>λ <> 0,15 93

Cyan 490 <>λ <> 0,26 75

Modra 460 <>λ <> 0,35 37

Septembra 2003 je Cree demonstriral novo vrsto modre LED, ki porabi 24 mW pri 20 miliamperih (mA). To je proizvedlo komercialno zapakirano belo svetlobo, ki je dalo 65 lm / W pri 20 mA, postane najsvetlejša bela LED, ki je komercialno na voljo v tem času, in več kot štirikrat bolj učinkovita kot standardne žarnice. Leta 2006 so pokazali prototip z rekordno belo LED svetlobno učinkovitostjo 131 lm / W pri 20 mA. Nichia Corporation je razvil belo LED z svetlobno učinkovitostjo 150 lm / W pri napetem toku 20 mA. [57] Cree's XLamp XM-L LED, ki so na tržišču na voljo v letu 2011, proizvajajo 100 lm / W pri njihovi polni moči 10 W in do 160 lm / W pri vhodni moči približno 2 W. Leta 2012 je Cree napovedal belo LED, ki je marca lansiral 254 lm / W, [58] in 303 lm / W. [59] Praktična splošna osvetlitev potrebuje visoko močne LED diode z eno vato ali več. Tipični obratovalni tokovi za takšne naprave se začnejo pri 350 mA.

Te učinkovitosti so samo za diode, ki oddajajo svetlobo in se držijo pri nizki temperaturi v laboratoriju. Ker svetleče diode, nameščene v realnem napajanju, delujejo pri višji temperaturi in izgubah voznika, so učinkovitosti v realnem svetu precej nižje. Preskušanje komercialnih LED žarnic, ki so bile izdelane za zamenjavo žarnic ali CFL, so pokazale, da je bila leta 2009 povprečna učinkovitost še vedno okoli 46 lm / W (testna zmogljivost je znašala od 17 lm / W do 79 lm / W). [60]

Učinkovitost droop [ uredi ]

Učinkovitost je zmanjšanje svetlobne učinkovitosti LED, saj se električni tok povečuje nad desetimi miljami.

Ta učinek je bil prvotno teoretiziran, da je povezan s povišanimi temperaturami. Znanstveniki so dokazali, da je resnično nasprotno: čeprav bi se življenje LED-dioda zmanjšalo, je pri visokih temperaturah nižja učinkovitost. [61] Mehanizem, ki je povzročil zmanjšanje učinkovitosti, je bil leta 2007 opredeljen kot Augerova rekombinacija , ki je bila vzeta z mešano reakcijo. [62] Leta 2013 je študija potrdila rekombinacijo Augerja kot vzrok za zmanjšanje učinkovitosti. [63]

Poleg tega, da so manj učinkovite, LED-diode z večjimi električnimi tokovi ustvarjajo višje nivoje toplote, ki ogrožajo življenjsko dobo LED. Zaradi tega povečanega ogrevanja pri višjih tokovih LED dioda visoke svetlosti ima industrijski standard delovanja le pri 350 mA, kar je kompromis med svetlobno močjo, učinkovitostjo in dolgo življenjsko dobo. [62] [64] [65] [66]

Možne rešitve [ uredi ]

Namesto povečanja trenutnih ravni se svetlost običajno poveča z združevanjem več LED v eno žarnico. Reševanje problema zmanjšanja učinkovitosti bi pomenilo, da bi LED žarnice za gospodinjstvo potrebovale manj LED, kar bi znatno zmanjšalo stroške.

Raziskovalci ameriškega pomorskega raziskovalnega laboratorija so našli način za zmanjšanje učinkovitosti. Ugotovili so, da kapljica nastane zaradi ne-sevalne Augerove rekombinacije vbrizganih nosilcev. Ustvarili so kvantne vdolbinice z mehkim omejevalnim potencialom za zmanjšanje ne-radiacijskih Augerjevih procesov. [67]

Raziskovalci na Tajvanski nacionalni centralni univerzi in podjetju Epistar Corp razvijajo način za zmanjšanje izkoristka z uporabo substratov iz keramičnih aluminijevih nitridov (AlN), ki so bolj toplotno prevodni od tržnega rabljenega safirja. Višja toplotna prevodnost zmanjšuje učinke samo-ogrevanja. [68]

Življenje in neuspeh [ uredi ]

Glavni članek: Seznam načinov izpada LED

Trdne naprave, kot so LED diode, so zelo omejene obrabe, če delujejo pri nizkih tokovih in pri nizkih temperaturah. Tipične življenjske dobe, ki so navedene od 25.000 do 100.000 ur, vendar lahko toplotna in trenutna nastavitev tolikokrat podaljšajo ali skrajšajo. [69]

Najpogostejši simptom odpovedi LED (in diode laserja ) je postopno zniževanje svetlobe in izguba učinkovitosti. Lahko se pojavijo tudi nenadni napadi, čeprav so redki. Zgodnje rdeče svetleče diode so bile opazne za njihovo kratko življenjsko dobo. Z razvojem visoko zmogljivih svetlečih diod, so naprave izpostavljene višjim temperaturnim razmeram in večjim gostoto toka od tradicionalnih naprav. To povzroča stres na materialu in lahko povzroči zgodnjo razgradnjo svetlobe. Za kvantitativno razvrstitev uporabne življenjske dobe na standardiziran način je bilo predlagano, da uporabljate L70 ali L50, ki so runtime (običajno v tisočih urah), pri katerih dani LED doseže 70% in 50% začetne svetlobne moči. [70]

Ker v večini prejšnjih svetlobnih virov (žarnice z žarilno nitko, žarnice za razsvetljavo in tiste, ki gorijo gorljivo gorivo, npr. Sveče in oljne svetilke), svetloba izvira iz ogrevanja, svetleče diode delujejo le, če so dovolj ohlajene. Proizvajalec običajno določi največjo temperaturo pri temperaturi 125 ° C ali 150 ° C, pri čemer je zaradi boljše življenjske dobe priporočljiva nižja temperatura. Pri teh temperaturah s sevanjem izgublja sorazmerno malo toplote, kar pomeni, da je svetlobni snop, ki ga ustvarja LED, ohlajen.

Odpadna toplota v močnostni LED (ki je od leta 2015 lahko manjša od polovice moči, ki jo porabi) se prenaša s prevodom skozi substrat in paket LED na hladilno telo , ki oddaja toploto okolju Zrak s konvekcijo. Previdnostni termični dizajn je zato bistven, ob upoštevanju toplotnih uporov svežnja LED, hladilnega telesa in vmesnika med obema. Srednje močne svetleče diode so pogosto zasnovane za spajkanje neposredno na tiskano vezje, ki vsebuje toplotno prevodno kovinsko plast. Visoko močne svetleče diode so pakirane v keramični embalaži velike velikosti, ki je zasnovana za pritrditev na kovinski hladilni sistem , pri čemer je vmesnik material z visoko toplotno prevodnostjo ( toplotna mast , material za fazno zamenjavo , termično prevodna blazinica ali termično lepilo ).

Če je svetilna luč s svetlečo diodo vgrajena v svetilko brez navzočnosti, ali svetilka se nahaja v okolju, kjer ni prostega kroženja zraka, se lahko LED pregreje, kar pomeni zmanjšano življenjsko dobo ali zgodnjo katastrofalno odpoved. Termična zasnova pogosto temelji na temperaturi okolice 25 ° C (77 ° F). LED, ki se uporabljajo v zunanjih aplikacijah, kot so prometni signali ali signalne lučke za pločnike, in v podnebjih, kjer je temperatura v svetlobni napetosti zelo visoka, lahko povzroči zmanjšano proizvodnjo ali celo napako. [71]

Ker je učinkovitost LED pri nizkih temperaturah višja, je LED tehnologija primerna za razsvetljavo supermarketov. [72] [73] [74] Ker svetleče diode proizvajajo manj odpadne toplote kot žarnice z žarilno nitko, lahko tudi pri uporabi v zamrzovalnikih prihrani tudi stroške hlajenja. Vendar so lahko bolj dovzetni za nastanek zmrzali in snega kot žarnice z žarilno nitko [71], zato so bili nekateri sistemi LED osvetlitve zasnovani z dodatnim krogom ogrevanja. Poleg tega so raziskave razvile tehnologije hladilnega telesa, ki bodo prenos toplote, proizvedene znotraj križišča, na ustrezna področja svetlobne napeljave. [75]

Barve in materiali [ uredi ]

Konvencionalne LED diode so izdelane iz različnih anorganskih polprevodniških materialov . V naslednji razpredelnici so prikazane razpoložljive barve z obsegom valovnih dolžin, padcem napetosti in materialom:


Barva Valovna dolžina [nm] Padec napetosti [ΔV] Polprevodniški material

Infrardeči Λ > 760 Δ V <> Gallijev arzenid (GaAs)
Aluminijev galijev arzenid (AlGaAs)

rdeča 610 <>λ <> 1,63 <δv><> Aluminijev galijev arzenid (AlGaAs)
Gallijev arzenid fosfid (GaAsP)
Aluminijev galijev indij fosfid (AlGaInP)
Galijev (III) fosfid (GaP)

Oranžna 590 <>λ <> 2.03 <δv><> Gallijev arzenid fosfid (GaAsP)
Aluminijev galijev indij fosfid (AlGaInP)
Galijev (III) fosfid (GaP)

Rumena 570 <>λ <> 2.10 <δv><> Gallijev arzenid fosfid (GaAsP)
Aluminijev galijev indij fosfid (AlGaInP)
Galijev (III) fosfid (GaP)

Zelena 500 <>λ <> 1,9 [76] <δ>V <> Tradicionalno zeleno:
Galijev (III) fosfid (GaP)
Aluminijev galijev indij fosfid (AlGaInP)
Aluminijev galijev fosfid (AlGaP)
Čisto zelena:
Indij galijev nitrid (InGaN) / galijev (III) nitrid (GaN)

Modra 450 <>λ <> 2.48 <δv><> Cink selenid (ZnSe)
Indijski galijev nitrid (InGaN)
Silicijev karbid (SiC) kot substrat
Silicijev (Si) kot substrata v razvoju

Violet 400 <>λ <> 2,76 <δ>V <> Indijski galijev nitrid (InGaN)

Vijolična Več tipov 2.48 <δv><> Dve modro / rdeče LED,
Modra z rdečim fosforjem,
Ali bele z vijolično plastiko

Ultravijolično Λ <> 3 <δ>V <> Indij galijev nitrid (InGaN) (385-400 nm)

Diamant (235 nm) [77]
Boron nitride (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]


Bela 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. [ citation needed ]

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. [ citation needed ]

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. [ citation needed ]

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. [ citation needed ]

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]

Vrste [ uredi ]

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 ur 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.