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

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 (LLE) and IES (the Illuminating Engineering Society of North America) color fidelity index (R). The average LLE of four road lighting standards of the USA and the UK decreases as R increases, but the optimal scotopic/photopic ratio increases slightly with R increases for a given correlated color temperature (CCT). The average LLE could reach the highest of 339  lm/W for R=70, 326  lm/W for R=80, and 309  lm/W for R=90 at CCT=5000  K. Six real pc-WLEDs with different R at CCT≈5000  K and with R≈70 at different CCT were demonstrated. Compared with current pc-WLEDs with yttrium aluminum garnet doped with Ce (YAG:Ce) phosphors, the average LLE 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 (520-530 nm) instead of YAG:Ce phosphors.