Spectral optimization of phosphor-coated white LED for road lighting based on the mesopic limited luminous efficacy and IES color fidelity index.

Abstract

The spectral optimization of phosphor-coated white LED (pc-WLED) with green and orange phosphors excited by blue chip for road lighting was investigated based on the mesopic limited luminous efficacy (LLEm) and IES (the Illuminating Engineering Society of North America) color fidelity index (Rf). The average LLEm of four road lighting standards of the USA and the UK decreases as Rf increases, but the optimal scotopic/photopic ratio increases slightly with Rf increases for a given correlated color temperature (CCT). The average LLEm could reach the highest of 339 lm/W for Rf=70, 326 lm/W for Rf=80, and 309 lm/W for Rf=90 at CCT=5000 K. Six real pc-WLEDs with different Rf at CCT≈5000 K and with Rf≈70 at different CCT were demonstrated. Compared with current pc-WLEDs with yttrium aluminum garnet doped with Ce3+ (YAG:Ce3+) phosphors, the average LLEm of six demonstrated pc-WLEDs will be over 5.0% and above. So, it is suggested that the road lighting should choose pc-WLEDs with high efficiency green phosphors (520-530nm) instead of YAG:Ce3+ phosphors.