Characterisation of total luminous flux measurements for light-emitting diodes

Abstract

Light-emitting diodes (LEDs) have found a use in various applications due to their compact size, durability and energy efficiency. Traditionally, due to low levels of illuminated light, LEDs have been mostly utilised as indication lamps for signalling purposes. The introduction of high power LEDs (specifically, phosphor-based white power LEDs) has been the drive behind the replacement of traditional incandescent and fluorescent lighting applications by their LED counterparts. This is due to LEDs and other solid-state lamps (SSLs) being far more energy efficient and durable. Moreover, SSL devices can be integrated into various shapes as a luminaire, thanks to its nature of being a tiny light source in discrete form. However, the optical and electrical nature of LEDs and SSLs is different from that of traditional light sources, like incandescent lamps. These distinguishing features of LEDs sets them apart from traditional light sources and means that the treatment of LEDs (in terms of measurement) must be carefully evaluated. Variations in measurements done by manufacturers, national laboratories and end-users have been reported. Some of these discrepancies in measurements are due to temperature drifts (which is expected for an LED, as it is a semi-conductor device), their directional or spatial nature, and LEDs being narrow band sources of light (resembling laser diodes). A method for measuring LED luminous flux has been studied and tested on a 50cm and a 200cm integrating sphere that makes use of readily available laboratory equipment. It is demonstrated that any laboratory set-up can be individually characterised to accommodate the measurement of LEDs with controlled accuracy. Measurement traceability is transferred from a reputable national laboratory institute of South Africa (NMISA). A lumen is realised from an illuminance standard that has been tested via global inter-laboratory comparisons against other international laboratories. A lumen realised using this method also traces to an NMISA giant primary standard (an absolute radiometer). This method eliminates the necessity of dealing with the issues that often arise when standard LED lamps are used as a reference when calibrating LED sources.

Lig-emissiediodes (LEDs) word gebruik in verskeie toepassings as gevolg van hul verenigbaarheid, duursaamheid en energiedoeltreffendheid. In die verlede is LEDs meestal gebruik as aanwyserlampe vir seindoeleindes as gevolg van hulle lae verligtingsvlakke. Die ontwikkeling van ho?drywing-LEDs (meer spesifiek, fosforgebaseerde wit LEDs) was die dryfveer vir die vervanging van konvensionele gloeilamp- en fluoresseerlampbeligting, in verskeie toepassings, deur toepaslike LEDs. Dit is omdat LEDs en ander vastetoestandligbronne (SSLs) baie meer energiedoeltreffend en duursaam is. Daarbenewens kan SSL-ligbronne gebruik word in verskeie vorms armature, omdat hulle klein bronne in kompakte vorm is. Die optiese en elektriese aard van LEDs en SSLs verskil egter van di? van konvensionele ligbronne, soos gloeilampe. Hierdie onderskeidende kenmerke van LEDs veroorsaak dat hierdie tipe ligbron heeltemal anders as konvensionele ligbronne toegepas moet word, met die gevolg dat die meting van LEDs anders hanteer en noukeurig ge?valueer moet word. Vervaardigers, nasionale laboratoriums en endgebruikers het variasies in resultate van metings deur hulle gedoen, aangemeld. Sommige van hierdie verskille is as gevolg van temperatuurdrywing (wat verwag kan word van 'n LED, omdat dit is 'n halfgeleiertoestel is), hulle gerigte of ruimtelike aard en omdat LEDs noubandligbronne kan wees (soortgelyk aan laserdiodes). 'n Metode is ondersoek en op die proef gestel op 'n 50 cm en 'n 200 cm integreersfeer onderskeidelik (wat toerusting gebruik wat geredelik beskikbaar in die laboratorium is). Daar is bevind dat enige laboratoriumopstelling individueel gekarakteriseer kan word om die meting van LEDs met akkuraatheid binne aanvaarbare toleransies te verkry. Die metingnaspeurbaarheid word oorgedra van 'n betroubare nasionale metrologie-instituut (NMISA). Die lumen word gerealiseer deur middel van 'n ligintensiteitstandaard wat geverifieer is deur middel van internasionale interlaboratoriumvergelykings met ander internasionale metrologie-institute. Tydens die lumenrealisering word daar ook naspeurbaarheid verkry na die nasionale prim?re standaard soos byvoorbeeld die absolute radiometer. Hierdie metode skakel die probleme uit wat dikwels ontstaan wanneer standaard LED-lampe as verwysing gebruik word tydens die kalibrasie van LED bronne.