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Dijowd li jarmi d-dawl (LED), li jarmi d-dawl meta l-elettroluminixxenza li tipproduċi l-elettriku titla '

Apr 21, 2017

Dijowd li jarmi d-dawl

Dijowd li jarmi d-dawl
RBG-LED.jpg LEDs blu, aħdar u ħomor f'kaxxa mxerrda ta '5 mm
Prinċipju ta 'ħidma Electroluminescence
Ivvintat H._J._Round (1907) [1]
Oleg Losev (1927) [2]
James R. Biard (1961) [3]
Nick Holonyak (1962) [4]
L-ewwel produzzjoni Ottubru 1962
Konfigurazzjoni tal-pin Anodu u katodu
Simbolu elettroniku
LED symbol.svg


Partijiet ta 'LED konvenzjonali. L-uċuħ tal-qiegħ ċatti tal-anvil u l-pożizzjoni mdaħħla ġewwa l-epossidu jaġixxu bħala ankri, biex jipprevjenu li l-kondutturi jinġibdu b'forza permezz ta 'pressjoni mekkanika jew vibrazzjoni.











Moderna retrofit LED bil-kamin E27 fil-bażi


Bozza tal-bozoz b'lura modernizzata b'forma ta ' bozza b'ilma ta' sħana tal- aluminju, koppla ta ' diffużjoni ħafifa u bażi tal- kamin E27 , bl-użu ta' provvista ta 'enerġija integrata li taħdem fuq vultaġġ tal-mains




Iċċekkja l-immaġni ta 'wiċċ immuntat LED





Dijowd li jarmi d-dawl ( LED ) huwa sors tad - dawl semikonduttur b'żewġ ċomb . Huwa dijodu ta ' junction p-n , li jarmi d-dawl meta jiġi attivat. [5] Meta tiġi applikata vultaġġ addattat għall-wajers, l- elettroni jistgħu jirkebblu ma 'toqob ta' l-elettroni ġewwa l-apparat, billi jirrilaxxaw l-enerġija fil-forma ta ' fotoni . Dan l-effett jissejjaħ elettroluminixxenza , u l-kulur tad-dawl (li jikkorrispondi għall-enerġija tal-foton) huwa ddeterminat mid- distakk tal- medda tal-enerġija tas-semikunduttur. L-LEDs huma tipikament żgħar (inqas minn 1 mm 2 ) u komponenti ottiċi integrati jistgħu jintużaw biex jagħtu forma lill -mudell tar - radjazzjoni . [6]

Jidher bħala komponenti elettroniċi prattiċi fl-1962, [7] l -LEDs l-aktar kmieni ħarġu dawl infra-aħmar ta 'intensità baxxa. L-LEDs infrared għadhom jintużaw ta 'spiss bħala elementi trasmittenti f'ċirkwiti ta' kontroll remot, bħal dawk f'kontrolli remoti għal varjetà wiesgħa ta 'elettronika għall-konsumatur. L-ewwel LEDs tad-dawl viżibbli kienu wkoll ta 'intensità baxxa u limitati għal aħmar. LEDs moderni huma disponibbli tul il-wavelengths viżibbli , ultraviolet u infra-aħmar , b'ħeffa kbira ħafna.

L-LED bikrija spiss jintuża bħala fanali indikaturi għal tagħmir elettroniku, li jieħu post bozoz inkandexxenti żgħar. Huma dalwaqt ġew ippakkjati f'lottijiet numeriċi fil-forma ta 'wirjiet ta' sekwenza ta ' sebgħa u kienu jidhru b'mod komuni f'xhur diġitali. Żviluppi reċenti fl-LEDs jippermettulhom li jintużaw fid-dwal ambjentali u tal-kompiti. L-LEDs ippermettew li jiġu żviluppati wirjiet u sensers ġodda, filwaqt li r-rati ta 'swiċċjar għoljin tagħhom jintużaw ukoll fit-teknoloġija tal-komunikazzjoni avvanzata.

L-LEDs għandhom bosta vantaġġi fuq sorsi ta 'dawl inkandexxenti inkluż konsum iktar baxx ta' enerġija, ħajja itwal, robustezza fiżika mtejba, daqs iżgħar, u bidla aktar mgħaġġla. Dijodi li jarmu d-dawl issa huma wżati f'applikazzjonijiet diversi bħad -dwal tal-avjazzjoni , il -bozoz ta 'quddiem awtomotivi , ir-reklamar, dawl ġenerali , sinjali tat-traffiku , flashes tal-kamera u wallpaper imdawla. Mill-2017, id-dwal tal-kamra tad-dwal tad-dwal LED huma irħas jew irħas minn sorsi ta ' lampa fluworexxenti kumpatta ta' produzzjoni komparabbli. [8] Huma wkoll ferm aktar effiċjenti fl-enerġija u, forsi, għandhom inqas tħassib ambjentali marbut mar-rimi tagħhom. [9] [10]


Werrej

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Storja [ editja ]

Skoperti u apparat bikri [ editja ]

Elettroluminixxenza ħadra minn punt ta 'kuntatt fuq kristall ta' SiC tirrikkera l-esperiment oriġinali tar-Round mill-1907.

L-elettroluminixxenza bħala fenomenu ġiet skoperta fl-1907 mill- espert Ingliż HJ Round tal -Labs Marconi , bl-użu ta 'kristall ta ' karbur tas-silikon u ditekter tal-qtates tal-qtates . [11] [12] L -inventur Russu Oleg Losev irrapporta dwar il-ħolqien tal-ewwel LED fl-1927. [13] Ir-riċerka tiegħu tqassmet f'ġurnali xjentifiċi Sovjetiċi, Ġermaniżi u Brittaniċi, iżda ma sar l-ebda użu prattiku tal-iskoperta għal diversi għexieren ta 'snin. [14] [15] Kurt Lehovec , Carl Accardo u Edward Jamgochian spjegaw dawn l-ewwel dijodi li jarmu d-dawl fl-1951 bl-użu ta 'apparat li juża kristalli SiC b'sors kurrenti ta' batterija jew ġeneratur tal-polz u b'paragun ma 'varjant, kristall pur Fl-1953. [16] [17]

Rubin Braunstein [18] tal- Korporazzjoni tar - Radju tal-Amerika rrapporta dwar l-emissjoni infra-aħmar minn arsenide tal-gallju (GaAs) u liegi semikondutturi oħra fl-1955. [19] Braunstein osservat infrared emissjoni ġġenerata minn strutturi sempliċi ta 'dijodu li jużaw antimonju gallju (GaSb), GaAs, indium Fosfid (InP), u silġ-ġermanju (SiGe) fit-temperatura tal-kamra u f'77 Kelvin.

Fl-1957, Braunstein wera wkoll li l-mezzi rudimentali jistgħu jintużaw għal komunikazzjoni mhux tar-radju f'distanza qasira. Kif innotat minn Kroemer [20] Braunstein "... waqqfet rabta ta 'komunikazzjoni ottika sempliċi: Mużika li ħarġet minn plejer ta' reġistrazzjoni intużat permezz ta 'elettronika adattata biex timmodifika l-kurrent ta' quddiem ta 'dija GaAs. Id-dawl emess ġie skopert b'diode PbS xi Distanza 'l bogħod Dan is-sinjal ingħata lil amplifikatur ta' l-awdjo u rritorna b'loudspeaker. Interċettazzjoni ta 'raġġ waqfet il-mużika. Kellna ħafna logħob divertenti ma' din is-setup. Din is-setup ippreżenat l-użu ta 'LED għal applikazzjonijiet ta' komunikazzjoni ottika.

A Texas Instruments SNX-100 GaAs LED li jinsab f'każ ta 'metall transistur TO-18.

F'Settembru 1961, waqt li kien qed jaħdem fuq Texas Instruments f'Dallas , Texas , James R. Biard u Gary Pittman skopra emissjoni ħafifa ta 'infrared qrib (900 nm) minn dijodu tal-mina li kienu bnew fuq substrat GaAs. [7] Sa Ottubru 1961, kienu urew emissjoni ħafifa effiċjenti u akkoppjar tas-sinjali bejn emissjoni tad-dawl ta 'GaAs pn junction u photodetector semikonduttur elettrikament iżolat. [21] Fit-8 ta 'Awwissu 1962, Biard u Pittman ressqu privattiva bit-titlu "Dijodu Radjanti Semikondutturi" ibbażata fuq is-sejbiet tagħhom, li ddeskrivew junction p-n mifrux bl-żingu LED b'kuntatt katodiku spazjat biex jippermetti emissjoni effiċjenti ta ' dawl infrared taħt Tħassir 'il quddiem . Wara li stabbilixxew il-prijorità tax-xogħol tagħhom ibbażat fuq notebooks ta 'l-inġinerija li kienu qabel is-sottomissjonijiet minn GE Labs, Labs tar- RCA Research, IBM Research Labs, Bell Labs u Lincoln Lab fil- MIT . ) Dijodu li jarmi d-dawl (US Patent US3293513 ), l-ewwel LED prattiku. [7] Immedjatament wara l-preżentazzjoni tal-privattiva, Texas Instruments (TI) beda proġett għall-manifattura ta 'diodes infrared. F'Ottubru 1962, TI ħabbret l-ewwel prodott LED kummerċjali (is-SNX-100), li impjegat kristall pur GaAs biex jarmi output tad-dawl ta '890 nm. [7] F'Ottubru 1963, TI ħabbret l-ewwel LED emisferiku kummerċjali, is-SNX-110. [22]

L-ewwel LED ta 'l-ispettru viżibbli (aħmar) ġie żviluppat fl-1962 minn Nick Holonyak, Jr waqt li kien qed jaħdem fuq General Electric . Holonyak l-ewwel irrapporta l-LED tiegħu fil-Ġurnal Applied Physics Letters fl-1 ta 'Diċembru, 1962. [23] [24] M. George Craford , [25] student gradwat ta' Holonyak, ivvintaw l-ewwel LED isfar u tejbu l-luminożità tal-aħmar u LED aħmar-oranġjo b'fattur ta 'għaxra fl-1972. [26] Fl-1976, TP Pearsall ħolqot l-ewwel LEDs ta' qawwa għolja u ta 'effiċjenza għolja għal telekomunikazzjonijiet ta' fibri ottiċi billi vvinta materjali semikondutturi ġodda speċifikament adattati għal wavelengths ta 'trażmissjoni ta' fibra ottika. [27]

Żvilupp kummerċjali inizjali [ editja ]

L-ewwel LED kummerċjali kienu komunement użati bħala sostituti għal lampi indikaturi inkandexxenti u tan-neon , u f'displejs ta 'seba' segment [28], l- ewwel f'tagħmir li jiswa ħafna bħal tagħmir tal-laboratorju u elettroniku, imbagħad aktar tard f'tagħmir bħal televiżjonijiet, radji, Kalkulaturi, kif ukoll arloġġi (ara l-lista ta ' użi tas-sinjali ). Sal-1968, LEDs viżibbli u infrared kienu jiswew ħafna, fl-ordni ta ' US $ 200 għal kull unità, u għalhekk kellhom ftit użu prattiku. [29] Il- Kumpanija Monsanto kienet l-ewwel organizzazzjoni li tipproduċi massa LEDs viżibbli, bl-użu tal-gallium arsenide phosphide (GaAsP) fl-1968 biex tipproduċi LEDs ħomor adattati għall-indikaturi. [29] Hewlett Packard (HP) introduċa LEDs fl-1968, inizjalment bl-użu ta 'GaAsP fornut minn Monsanto. Dawn l-LEDs ħomor kienu jleqqu biżżejjed biss għall-użu bħala indikaturi, minħabba li l-ħruġ tad-dawl ma kienx biżżejjed biex idawwal żona. Il-qari fil-kalkulaturi kien tant żgħir li l-lentijiet tal-plastik inbnew fuq kull numru biex ikunu jistgħu jinqraw. Aktar tard, kuluri oħra saru disponibbli b'mod wiesa 'u dehru f'tagħmir u tagħmir. Fis-snin 70, il-Fairchild Optoelectronics ipproduċiet apparati LED b'suċċess kummerċjali b'inqas minn ħames ċenteżmi. Dawn il-mezzi użaw ċipep tas-semikondutturi komposti fabbrikati bil- proċess ċatt ivvintat minn Dr Jean Hoerni fi Fairchild Semiconductor . [30] [31] Il-kombinazzjoni ta 'proċessar ċatt għall-fabbrikazzjoni taċ-ċippa u metodi ta' ippakkjar innovattivi ppermettew lit-tim f'Firchild mmexxi mill-pijunier ta 'optoelectronics Thomas Brandt biex jikseb it-tnaqqis fl-ispejjeż meħtieġa. [32] Dawn il-metodi jibqgħu jintużaw mill-produtturi tal-LEDs. [33]

Displej LED ta 'kalkulatur xjentifiku TI-30 (ca. 1978), li juża lenti tal-plastik biex iżid id-daqs ta' ċifra viżibbli

Il-biċċa l-kbira tal-LEDs saru fil-pakketti T1 u 3mm T1 komuni ħafna ħafna, iżda b'żieda fil-produzzjoni tal-qawwa, kiber dejjem aktar neċessarju biex jitfa 'sħana żejda biex iżżomm l-affidabilità [34] hekk pakketti aktar kumplessi ġew adattati għal dissipazzjoni effiċjenti tas- . Il-pakketti għal LED ta 'qawwa għolja ta' l-aktar teknoloġija għolja għandhom ftit xebh mal-LEDs bikrija.

Blue LED [ editja ]

L-LEDs l-ewwel ġew żviluppati minn Herbert Paul Maruska f'RCA fl-1972 bl-użu ta 'nitrude tal-gallju (GaN) fuq sustrat taż-żaffir. [35] [36] It -tipi SiC kienu l-ewwel mibjugħa kummerċjalment fl- Istati Uniti minn Cree fl-1989. [37] Madankollu, l-ebda wieħed minn dawn l-LEDs blu inizjali ma kien qawwi ħafna.

L-ewwel LED blu ta 'qawwa għolja tidher minn Shuji Nakamura ta' Nichia Corporation fl-1994 u kienet ibbażata fuq InGaN . [38] [39] B'mod parallel, Isamu Akasaki u Hiroshi Amano f'Nagoya kienu qed jaħdmu fuq l-iżvilupp tan-nukarazzjoni GaN importanti fuq is-sustrati taż-żaffir u d-dimostrazzjoni ta 'doping tat-tip p ta' GaN. Nakamura, Akasaki u Amano ingħataw il- Premju Nobel 2014 fil-fiżika għax-xogħol tagħhom. [40] Fl-1995, Alberto Barbieri fil-Laboratorju Universitarju ta 'Cardiff (GB) investiga l-effiċjenza u l-affidabbiltà ta' LEDs ta 'qawwa għolja u wera LED ta' "kuntatt trasparenti" bl-użu ta ' indium tin oxide (ITO) fuq (AlGaInP / GaAs).

Fl-2001 [41] u l-2002, [42] il- proċessi għat-tkabbir ta ' nitride tal - gallju (GaN) LEDs fuq silikon intwerew b'suċċess. F'Jannar 2012, Osram wera qawwa għolja InGaN LEDs imkabbra fuq sottostrati tas-silikon kummerċjalment. [43]

LEDs bojod u l-avvanz ta 'illuminazzjoni [ editja ]

Il-kisba ta 'effiċjenza għolja fl-LEDs blu kienet segwita malajr bl-iżvilupp tal-ewwel LED abjad . F'dan il-mezz Y
3 Al
5 O
12 : Kisi ta 'fosfru ta' Ce (magħruf bħala " YAG ") fuq l-emitter jassorbi ftit mill-emissjoni blu u jipproduċi dawl isfar permezz ta ' fluworexxenza . Il-kombinazzjoni ta 'dak isfar bid-dawl blu li jifdal jidher abjad għall-għajn. Madankollu, bl-użu ta ' fosfri differenti (materjali fluworexxenti) sar ukoll possibbli li minflok jipproduċi dawl aħdar u aħmar permezz ta' fluworexxenza. It-taħlita li tirriżulta ta 'aħmar, aħdar u blu mhix biss perċepita mill-bnedmin bħala dawl abjad iżda hija superjuri għall-illuminazzjoni f'termini ta' rendering tal- kulur , filwaqt li wieħed ma jistax japprezza l-kulur ta 'oġġetti aħmar jew aħdar illuminati biss bl-isfar (u blu li jifdal) Tul ta 'mewġ mill-fosfru YAG.

Illustrazzjoni tal -liġi ta ' Haitz , li turi titjib fil-produzzjoni tad-dawl għal kull LED matul iż-żmien, bi skala logaritmika fuq l-assi vertikali

L-ewwel LED bojod kienu għaljin u ineffiċjenti. Madankollu, il-produzzjoni tad-dawl ta 'LEDs żdiedet b'mod esponenzjali , u rduppjar seħħ madwar kull 36 xahar mis-sittinijiet (simili għal -liġi ta' Moore ). Din it-tendenza hija ġeneralment attribwita għall-iżvilupp parallel ta 'teknoloġiji semikondutturi oħra u avvanzi fl-ottika [ ċitazzjoni meħtieġa ] u xjenza tal-materjali u ġiet imsejħa liġi Haitz wara Dr Roland Haitz. [44]

L-output tad-dawl u l-effiċjenza ta 'LEDs blu u kważi-ultravjola żdiedu minħabba li l-ispiża ta' apparati affidabbli waqgħet: dan wassal għall-użu ta 'LEDs ta' dawl qawwi (relattivament) qawwija għall-iskop ta 'illuminazzjoni li qed jissostitwixxu dawl inkandexxenti u fluworexxenti. [45] [46]

Intwerew LEDs bojod sperimentali li jipproduċu iktar minn 300 lumens kull watt ta 'elettriku; Xi wħud jistgħu jdumu sa 100,000 siegħa. [47] Meta mqabbel mal-bozoz inkandexxenti, din mhix biss żieda kbira fl-effiċjenza elettrika iżda - matul iż-żmien - spiża simili jew inqas għal kull bozza. [48]

Prinċipju ta 'ħidma [ editja ]

Il-ħidma interna ta 'LED, li turi ċirkwit (fuq) u dijagramma tal-faxxa (qiegħ)

Il-junction PN jista 'jikkonverti l-enerġija ħafifa assorbita f'kurrent elettriku proporzjonali. L-istess proċess jinqaleb hawnhekk (jiġifieri l-junction PN jarmi d-dawl meta tiġi applikata enerġija elettrika għaliha). Dan il-fenomenu huwa ġeneralment imsejjaħ elettroluminixxenza , li jista 'jiġi definit bħala l-emissjoni ta' dawl minn semikonduttur taħt l-influwenza ta ' kamp elettriku . It-trasportaturi tal-ħlas jikkumbinaw f'punt PN preġudikat bi preġudizzju billi l-elettroni jaqsmu mir-reġjun N u jikkombinaw mat-toqob eżistenti fir-reġjun P. L-elettroni b'xejn huma fil- faxxa tal- konduzzjoni tal-livelli tal-enerġija, filwaqt li toqob huma fil- medda tal-enerġija tal- valenza. Għalhekk il-livell ta 'l-enerġija tat-toqob ikun inqas mill-livelli ta' l-enerġija ta 'l-elettroni. Uħud mill-porzjonijiet tal-enerġija għandhom jinħallu sabiex jiġu rikombinati l-elettroni u t-toqob. Din l-enerġija toħroġ fil-forma ta 'sħana u dawl.

L-elettroni jxerrdu l-enerġija fil-forma ta 'sħana għal silikon u dijodi tal-ġermanju imma f'semikondutturi tal-gallium arsenide phosphide (GaAsP) u gallium phosphide (GaP), l-elettroni jiddisspaċu l-enerġija billi jarmu fotoni . Jekk is-semikonduttur ikun trasluċenti, il-junction isir is-sors tad-dawl hekk kif joħroġ, u b'hekk isir dijodu li jarmi d-dawl, iżda meta l-junction ikun imxaqilb'lura ma jkun hemm l-ebda dawl prodott mill-LED u jekk il-potenzjal ikun kbir biżżejjed, L-apparat se jkun bil-ħsara.

Teknoloġija [ editja ]

Dijagramma IV għal dijodu . L-LED se jibda jarmi d-dawl meta jiġu applikati aktar minn 2 jew 3 volts. Ir-reġjun ta 'preġudizzju b'lura juża skala vertikali differenti mir-reġjun ta' preġudizzju 'l quddiem, sabiex juri li l-kurrent tat-tnixxija huwa kważi kostanti b'vultaġġ sakemm isseħħ it-tqassim. Fil-preġudizzju 'l quddiem, il-kurrent huwa żgħir iżda jiżdied b'mod esponenzjali mal-vultaġġ.

Fiżika [ editja ]

L-LED jikkonsisti f'ċippa ta 'materjal semikonduttiv imdewweb b'impuritajiet biex tinħoloq junction pn . Bħal f'diodi oħra, il-flussi tal-kurrent faċilment minn naħa-p, jew anodu , għan-naħa-n, jew katodu, iżda mhux fid-direzzjoni inversa. It-trasportaturi tad - debituri - elettroni u toqob - jidħlu fil-junction minn elettrodi b'vultaġġi differenti. Meta elettroni jissodisfa toqba, jaqa 'f'livell ta' enerġija aktar baxx u jirrilaxxa l- enerġija fil-forma ta ' foton .

Il- wavelength tad-dawl mitfugħ, u għalhekk il-kulur tiegħu, jiddependi fuq l-enerġija tal- lakuna tal- faxxa tal-materjali li jiffurmaw il- junction pn . Fid - dijodi tas-silikon jew tal- ġermanju , l-elettroni u t-toqob normalment jikkomombinaw permezz ta 'transizzjoni mhux radjanti , li ma tipproduċix emissjoni ottika, minħabba li dawn huma materjali indiretti . Il-materjali wżati għall-LED għandhom vojt tal-medda diretta b'enerġiji li jikkorrispondu ma 'dawl kważi infrared, viżibbli, jew kważi-ultravjola.

L-iżvilupp tal-LED beda b'apparat infra-aħmar u aħmar magħmul b'armju tal-gallju . L-avvanzi fix- xjenza tal-materjali ppermettew li jsiru apparati b'tul ta 'mewġa dejjem iqsar, li jarmu d-dawl f'varjetà ta' kuluri.

L-LEDs normalment ikunu mibnija fuq sottostrat ta 'tip-n, b'elettrodu mwaħħal mas-saff tat-tip p-depożitat fuq il-wiċċ tiegħu. Is-sustrati tat-tip P, filwaqt li huma inqas komuni, iseħħu wkoll. Ħafna LED kummerċjali, speċjalment GaN / InGaN, jużaw ukoll substrat taż-żaffir .

Indiċi refrattiv [ editja ]

Eżempju idealizzat ta 'koni ta' emissjoni ħafifa f'semikonduttur kwadru sempliċi, għal żona ta 'emissjoni ta' sors ta 'punt wieħed. L-illustrazzjoni tax-xellug hija għal wejfer trasluċidu, filwaqt li l-illustrazzjoni t-tajba turi l-half-cones iffurmati meta s-saff tal-qiegħ huwa opak. Id-dawl fil-fatt jitfa 'bl-istess mod fid-direzzjonijiet kollha mill-punt tas-sors, iżda jista' jaħrab biss perpendikolari għall-wiċċ tas-semikundutturi u xi gradi għan-naħa, li tidher mill-forom tal-kon. Meta jinqabeż l-angolu kritiku, il-fotoni huma riflessi internament. Iż-żoni bejn il-koni jirrappreżentaw l-enerġija ħafifa maqbuda moħlija bħala sħana. [49] Il-biċċa l-kbira tal-materjali użati għall-produzzjoni tal-LED għandhom indiċi rifrattivi għoljin ħafna. Dan ifisser li ħafna mid-dawl se jiġi rifless lura fil-materjal fl-interface tal-materjal / arja tal-wiċċ. Għalhekk, l- estrazzjoni tad-dawl fl-LEDs hija aspett importanti tal-produzzjoni tal-LED, soġġetta għal ħafna riċerka u żvilupp. Il-koni ta 'emissjoni ħafifa ta' wejfer LED reali huma ħafna aktar kumplessi minn emissjoni tad-dawl ta 'punt ta' sors wieħed. Iż-żona ta 'emissjoni ħafifa hija tipikament pjan bi-dimensjonali bejn il-wejfers. Kull atomu madwar dan il-pjan għandu sett individwali ta 'koni ta' emissjoni. It-tfassil tal-biljuni ta 'koni li jikkoinċidu huwa impossibbli, għalhekk din hija dijagramma ssimplifikata li turi l-estensjonijiet tal-koni kollha tal-emissjoni kkombinati. Il-koni tal-ġenb akbar huma maqtugħin biex juru l-karatteristiċi interni u jnaqqsu l-kumplessità tal-immaġini; Huma jkunu estiżi għat-truf opposti tal-pjan ta 'emissjoni bidimensjonali.

Semikondutturi mhux miksija vojta bħas- silikon juru indiċi rifrattiv għoli ħafna meta mqabbel ma 'arja aperta, li tipprevjeni l-passaġġ ta' fotoni li jaslu f'angoli qawwija relattivi għall-wiċċ li jikkuntattja l-arja tas-semikunduttur minħabba riflessjoni interna totali . Din il-proprjetà tolqot kemm l-effiċjenza tal-emissjoni tad-dawl tal-LEDs kif ukoll l-effiċjenza tal-assorbiment tad-dawl taċ- ċelloli fotovoltajċi . L-indiċi refrattiv tas-silikon huwa 3.96 (f'590 nm), [50] filwaqt li l-arja hija 1.0002926. [51]

B'mod ġenerali, ċippa semikonduttur LED b'wiċċ ċatt mhux miksi se toħroġ dawl perpendikulari biss għall-wiċċ tas-semikondutturi, u ftit gradi lejn il-ġenb, f'forma ta 'koni magħrufa bħala l- koni tad - dawl , koni tad-dawl [52] jew il- ħrib Konu . [49] L- angolu massimu ta 'inċidenza jissejjaħ l- angolu kritiku . Meta jinqabeż dan l-angolu, il-fotoni m'għadhomx jaħarbu mis-semikondutturi imma minflok huma riflessi internament ġewwa l-kristall semikondutturi bħallikieku kienet mera . [49]

Ir-riflessi interni jistgħu jaħarbu minn uċuh kristallini oħra jekk l-angolu ta 'l-inċidenza jkun baxx biżżejjed u l-kristall ikun trasparenti biżżejjed biex ma jassorbix mill-ġdid l-emissjoni tal-foton. Imma għal LED kwadru sempliċi b'uċuħ angolati ta '90 grad fuq in-naħat kollha, l-uċuħ kollha jaġixxu bħala mirja ta' angolu ugwali. F'dan il-każ, il-biċċa l-kbira tad-dawl ma jistax jaħrab u jintilef bħala sħana skartata fil-kristall. [49]

Wiċċ ta 'ċippa konvolut b'fatturi angolati simili għal ġojjell jew lenti tal-Fresnel jista' jżid il-ħruġ tad-dawl billi jippermetti li d-dawl jiġi emess perpendikulari għas-superfiċje taċ-ċippa sakemm il-ġenb tal-punt tal-emissjoni tal-foton. [53]

Il-forma ideali ta 'semikonduttur b'tagħbija massima tad-dawl tkun mikrosfera bl-emissjoni tal-foton li sseħħ fiċ-ċentru eżatt, bl-elettrodi li jippenetraw fiċ-ċentru biex jikkuntattjaw fil-punt tal-emissjoni. Ir-raġġi tad-dawl kollha li joħorġu miċ-ċentru jkunu perpendikolari għall-wiċċ kollu tal-isfera, li tirriżulta f'ebda riflessjoni interna. Semikonduttur emisferiku jaħdem ukoll, bil-wiċċ ċatt li jservi bħala mera għal fotoni mferrxa lura. [54]

Kisi tat-transizzjoni [ editja ]

Wara d-doping tal- wejfer , dan jinqata 'f'mutur individwali. Kull die normalment tissejjaħ ċippa.

Bosta ċipep tas-semikondutturi LED huma inkapsulati jew ippakkjati f'folji tal-plastik iffurmati ċari jew ikkuluriti. Il-qoxra tal-plastik għandha tliet għanijiet:

  1. L-immontar taċ-ċippa tas-semikondutturi f'tagħmir huwa aktar faċli biex jitwettaq.

  2. Il-wajers elettriċi fraġli ċkejkna huma sostnuti fiżikament u protetti mill-ħsara.

  3. Il-plastik jaġixxi bħala intermedjarju refrattiv bejn is-semikondutturi ta 'indiċi relattivament għoli u l-arja baxxa b'inqas indiċi. [55]

It-tielet karatteristika tgħin biex iżżid l-emissjoni tad-dawl mis-semikondutturi billi taġixxi bħala lenti li tiddissemina, li tippermetti li d-dawl jiġi emmess f'angolu ta 'inċidenza ferm ogħla mill-kon tad-dawl mill-ċippa vojta jista' jarmi waħdu.

Effiċjenza u parametri operattivi [ editja ]

L-LED indikaturi tipiċi huma ddisinjati biex jaħdmu b'mhux aktar minn 30-60 milliwatt (mW) ta 'enerġija elettrika. Madwar l-1999, Philips Lumileds introduċiet qawwa LED li tista 'tintuża kontinwament f'watt wieħed. Dawn l-LEDs użaw daqsijiet ferm akbar tas-semikondutturi biex jimmaniġġjaw id-dħul qawwi tal-qawwa. Ukoll, il-semi-konduttur imut kien immuntat fuq biċċiet tal-metall li jippermettu t-tneħħija tas-sħana mill-die LED.

Wieħed mill-vantaġġi ewlenin tas-sorsi tad-dawl ibbażati fuq il-LED huwa effikaċja luminuża għolja. L-LEDs bojod irranġaw malajr u qabżu l-effikaċja tas-sistemi standard tad-dwal inkandexxenti. Fl-2002, Lumileds għamel LEDs ta 'ħames watt disponibbli b'effikaċja luminuża ta' 18-22 lumens kull watt (lm / W). Għal paragun, bozza inkandexxenti konvenzjonali ta '60-100 watts tarmi madwar 15 lm / W, u dwal fluworexxenti standard jarmu sa 100 lm / W.

Mill-2012, Philips kien kiseb l-effikaċja li ġejja għal kull kulur. [56] Il-valuri tal-effiċjenza juru l-qawwa tal-fjamma tal-fwar għal kull qawwa elettrika. Il-valur tal-effikaċja ta 'lumen-per-watt jinkludi karatteristiċi tal-għajn tal-bniedem u huwa derivat bl-użu tal- funzjoni tal-luminożità .


Kulur Firxa tal-wavelength (nm) Koeffiċjent ta 'effiċjenza tipiku Effikaċja tipika ( lm / W )

Aħmar 620 <>λ <> 0.39 72

Aħmar oranġjo 610 <>λ <> 0.29 98

Aħdar 520 <>λ <> 0.15 93

Cyan 490 <>λ <> 0.26 75

Blu 460 <>λ <> 0.35 37

F'Settembru 2003, ġie muri tip ġdid ta 'LED blu minn Cree li tikkonsma 24 mW f'20 milliamperes (mA). Dan ipproduċa dawl abjad imballat kummerċjalment u ta '65 lm / W f'20 mA, li sar l-LED abjad l-aktar qawwi disponibbli kummerċjalment dak iż-żmien, u aktar minn erba' darbiet aktar effiċjenti mill-inkandexxenti standard. Fl-2006, huma wrew prototip b'effikaċja luminuża LED abjad rekord ta '131 lm / W f'20 mA. Nichia Corporation żviluppat LED abjad b'effiċjenza luminuża ta '150 lm / W b'kurrent' il quddiem ta '20 mA. [57] L-LEDs XLamp XM-L ta 'Cree, kummerċjalment disponibbli fl-2011, jipproduċu 100 lm / W bl-enerġija sħiħa tagħhom ta' 10 W, u sa 160 lm / W f'madwar qawwa input ta '2 W. Fl-2012, Cree ħabbar LED abjad li jagħti 254 lm / W, [58] u 303 lm / W f'Marzu 2014. [59] Dawl ġenerali prattiku jeħtieġ LEDs ta 'qawwa għolja, ta' watt wieħed jew aktar. Il-kurrenti tat-tħaddim tipiċi għal tali apparat jibdew f'temperatura ta '350 mA.

Dawn l-effiċjenzi huma għad-dijodu li tarmi d-dawl biss, miżmuma f'temperatura baxxa f'laboratorju. Peress li l-LEDs installati f'fittings reali joperaw f'temperatura ogħla u bit-telf tas-sewwieq, l-effiċjenzi fid-dinja reali huma ħafna aktar baxxi. L- ittestjar tad- Dipartiment ta 'l-Enerġija ta' l-Istati Uniti (DOE) ta 'lampi kummerċjali LED iddisinjati biex jissostitwixxu bozoz inkandexxenti jew CFLs urew li l-effikaċja medja kienet għadha madwar 46 lm / W fl-2009 (prestazzjoni ttestjata varjat minn 17 lm / W sa 79 lm / W). [60]

Efficiency droop [ editja ]

It-tnaqqis fl-effiċjenza huwa t-tnaqqis fl-effiċjenza luminuża tal-LEDs hekk kif il -kurrent elettriku jiżdied aktar minn għexieren ta 'milliamperes.

Dan l-effett ġie inizjalment teorizzat biex ikun relatat ma 'temperaturi elevati. Ix-xjentisti wrew li l-oppost ikun veru: għalkemm il-ħajja ta 'LED tkun imqassra, id-droop ta' effiċjenza huwa inqas sever f'temperaturi elevati. [61] Il-mekkaniżmu li jikkawża tnaqqis fl-effiċjenza ġie identifikat fl-2007 bħala recombination tal-Auger , li ttieħed b'reazzjoni mħallta. [62] Fl-2013, studju kkonferma r-rikombinazzjoni ta 'Auger bħala l-kawża tad-droop ta' l-effiċjenza. [63]

Minbarra li huma inqas effiċjenti, l-LEDs li jaħdmu fuq kurrenti elettriċi ogħla joħolqu livelli ogħla ta 'sħana li jikkompromettu l-ħajja tal-LED. Minħabba dan it-tisħin miżjud f'livelli ogħla, LEDs ta 'qawwa għolja għandhom standard industrijali li jopera biss 350 mA, li huwa kompromess bejn il-produzzjoni tad-dawl, l-effiċjenza u l-lonġevità. [62] [64] [65] [66]

Soluzzjonijiet possibbli [ editja ]

Minflok ma żżid il-livelli attwali, il-luminanza normalment tiżdied billi tgħaqqad LEDs multipli f'bozza waħda. Is-soluzzjoni tal-problema tad-droop ta 'l-effiċjenza tkun tfisser li l-bozoz tad-dawl tad-dar LED jkollhom inqas LEDs, li jnaqqsu b'mod sinifikanti l-ispejjeż.

Ir-riċerkaturi fil- Laboratorju tar-Riċerka Navali ta 'l-Istati Uniti sabu mod kif titnaqqas id-droop ta' l-effiċjenza. Huma sabu li d-droop ġej minn rikombinazzjoni ta 'Auger mhux radjoattiva tat-trasportaturi injettati. Huma ħolqu bjar quantum b'potenzjal ta 'reżinjar artab biex inaqqsu l-proċessi ta' Auger mhux radjanti. [67]

Ir-riċerkaturi fl -Università Ċentrali Nazzjonali ta 'Tajwan u l- Epistar Corp qed jiżviluppaw mod kif titnaqqas id- drogi ta ' l-effiċjenza bl-użu ta 'sottostrati taċ-ċeramika tan-nitrude ta' l-aluminju (AlN), li huma aktar konduttivi termalment mis-sapphire użat kummerċjalment. Il-konduttività termali ogħla tnaqqas l-effetti li jsaħħnu waħedhom. [68]

Lifetime u falliment [ editja ]

L-Artikolu prinċipali: Lista ta 'modi ta' falliment LED

L-istrumenti tal-istat solidu bħall-LEDs huma soġġetti għal xedd u kedd limitat ħafna jekk jitħaddmu bi kurrenti baxxi u f'temperaturi baxxi. Ħajtiet tipiċi kkwotati huma 25,000 sa 100,000 siegħa, iżda s-sħana u s-settings attwali jistgħu jestendu jew iqassru dan iż-żmien b'mod sinifikanti. [69]

L-aktar sintomu komuni tal-ħsara tal-LED (u tal- laser diode ) huwa t-tnaqqis gradwali tal-produzzjoni tad-dawl u t-telf tal-effiċjenza. Jistgħu jseħħu wkoll fallimenti f'daqqa, għalkemm rari. LED aħmar bikri kienu notevoli għall-ħajja qasira ta 'servizz tagħhom. Bl-iżvilupp ta 'LED ta' qawwa għolja, il-mezzi huma soġġetti għal temperaturi ta 'junction ogħla u densitajiet ta' kurrent ogħla minn apparati tradizzjonali. Dan jikkawża stress fuq il-materjal u jista 'jikkawża degradazzjoni ta' ħruġ bid-dawl kmieni. Biex tiġi kklassifikata b'mod kwantitattiv il-ħajja utli b'mod standardizzat ġie ssuġġerit li tuża L70 jew L50, li huma r-timeshops (tipikament mogħtija f'eluf ta 'sigħat) li fihom LED partikolari jilħaq is-70% u 50% tad-dawl inizjali, rispettivament. [70]

Billi fil-biċċa l-kbira tas-sorsi ta 'dawl preċedenti (bozoz inkandexxenti, bozoz ta' skarika, u dawk li jaħarqu karburant kombustibbli, eż. Xemgħat u lampi taż-żejt) ir-riżultati tad-dawl jirriżultaw mis-sħana, LEDs joperaw biss jekk jinżammu friski biżżejjed. Il-manifattur spiss jispeċifika temperatura ta 'junction massimu ta' 125 jew 150 ° C, u temperaturi aktar baxxi huma rakkomandabbli fl-interess ta 'ħajja twila. F'dawn it-temperaturi, sħana relattivament baxxa tintilef mir-radjazzjoni, li jfisser li r-raġġ ta 'dawl iġġenerat minn LED jibred.

Is-sħana mormija f'LED ta 'qawwa għolja (li mill-2015 tista' tkun inqas minn nofs is-saħħa li tikkonsma) titwassal permezz tal-konduzzjoni permezz tas-substrat u l-pakkett tal-LED għal sink tas - sħana , li tagħti s-sħana lill-ambjent Arja b'konvezzjoni. Id-disinn termiku bir-reqqa huwa, għalhekk, essenzjali, waqt li jitqiesu r -reżistenzi termali tal-pakkett tal-LED, is-sink tas-sħana u l-interface bejn it-tnejn. L-LED ta 'qawwa medja ta' spiss huma ddisinjati biex ikunu ssaldjati direttament f'karta ta ' ċirkwit stampat li jkun fih saff tal-metall konduttiv termalment. LEDs ta 'qawwa għolja huma ppakkjati f'pakketti ta' ċeramika ta 'żona kbira ddisinjati biex jitwaħħlu ma' sink tas-sħana tal-metall, li l-interface huwa materjal b'konduttività termali għolja ( grass termali , materjal li jibdel il-fażi , kuxxinett konduttiv termalment jew kolla termika ).

Jekk lampa b'bażi ta 'LED hija installata f'luminatur mhux ventilat, jew luminaire jinsab f'ambjent li m'għandux ċirkolazzjoni ta' arja libera, l-LED x'aktarx jisħon iżżejjed, li jirriżulta f'ħajja mnaqqsa jew falliment katastrofiku bikri. Id-disinn termiku ħafna drabi huwa bbażat fuq temperatura ambjentali ta '25 ° C (77 ° F). LEDs użati f'applikazzjonijiet ta 'barra, bħal sinjali tat-traffiku jew dwal tas-sinjali tal-bankina, u fi klimi fejn it-temperatura ġewwa l-apparat tad-dawl gets għoli ħafna, jistgħu jesperjenzaw produzzjoni mnaqqsa jew saħansitra falliment. [71]

Peress li l-effikaċja tal-LED hija ogħla f'temperaturi baxxi, it-teknoloġija tal-LED hija adattata tajjeb għad-dawl tal- friża tas- supermarket. [72] [73] [74] Minħabba li l-LEDs jipproduċu anqas sħana mill-bozoz inkandexxenti, l-użu tagħhom fil-friżers jista 'jiffranka wkoll fuq spejjeż ta' refriġerazzjoni. Madankollu, jistgħu jkunu aktar suxxettibbli għall-ġlata u l-għeruq tas-silġ minn bozoz inkandexxenti, [71] u għalhekk xi sistemi tad-dawl LED ġew iddisinjati b'ċirkwit ta 'tisħin miżjud. Barra minn hekk, ir-riċerka żviluppat teknoloġiji tas-sħana li se jittrasferixxu s-sħana prodotta fil-junction għal żoni xierqa tal-muntaġġ tad-dawl. [75]

Kuluri u materjali [ editja ]

L-LEDs konvenzjonali huma magħmula minn varjetà ta ' materjali semikondutturi inorganiċi. It-tabella li ġejja turi l-kuluri disponibbli bil-medda tat-tul ta 'mewġ, il-waqa' tal-vultaġġ u l-materjal:


Kulur Wavelength [nm] Qatra tal-vultaġġ [ΔV] Materjal semikunduttiv

Infrared Λ > 760 Δ V <> Arsenide tal-gallju (GaAs)
Aluminum gallium arsenide (AlGaAs)

Aħmar 610 <>λ <> 1.63 <δ>V <> Aluminum gallium arsenide (AlGaAs)
Fossidu tal-arsenju tal-gallju (GaAsP)
Aluminum gallium indium phosphide (AlGaInP)
Fosfru tal-gallju (III) (GaP)

Oranġjo 590 <>λ <> 2.03 <δ>V <> Fossidu tal-arsenju tal-gallju (GaAsP)
Aluminum gallium indium phosphide (AlGaInP)
Fosfru tal-gallju (III) (GaP)

Isfar 570 <>λ <> 2.10 <δ>V <> Fossidu tal-arsenju tal-gallju (GaAsP)
Aluminum gallium indium phosphide (AlGaInP)
Fosfru tal-gallju (III) (GaP)

Aħdar 500 <>λ <> 1.9 [76] <δ>V <> Ħodor tradizzjonali:
Fosfru tal-gallju (III) (GaP)
Aluminum gallium indium phosphide (AlGaInP)
Aluminum gallium phosphide (AlGaP)
Pura aħdar:
Indium gallium nitrid (InGaN) / Gallium (III) nitrude (GaN)

Blu 450 <>λ <> 2.48 <δ>V <> Selenide taż-żingu (ZnSe)
Indium gallium nitrid (InGaN)
Karbur tas-silikon (SiC) bħala substrat
Silicon (Si) bħala sottostrat taħt żvilupp

Vjola 400 <>λ <> 2.76 <δ>V <> Indium gallium nitrid (InGaN)

Vjola Tipi multipli 2.48 <δ>V <> LEDs doppji blu / aħmar,
Blu bil-fosfru aħmar,
Jew abjad bil-plastik vjola

Ultravjola Λ <> 3 < δ="">V < 4.1=""> Indium gallium nitride (InGaN) (385-400 nm)

Diamond (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]


Abjad 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]

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 3,000
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.