The dependence of luminous efficacy on phosphor concentration and thickness for high-power white light-emitting-diode (WLED) lamps is investigated by employing three-dimensional ray-tracing simulations. The simulations show that the brightness or luminous efficacy of WLED lamps highly depends on the combination of phosphor concentration and phosphor thickness (or phosphor-matrix composite volume). The package with lower concentration and higher phosphor thickness has higher luminous efficacy because the light trapping efficiency is lower with the low phosphor concentration. At the correlated color temperature (CCT) value of around 4000 K, ray-tracing simulation and experimental results show 20% and 23% improvement in lumen, respectively, with a 1.8-mm-phosphor package over a 0.8-mm-phosphor package. A package with convex lens can improve the lumen output over flat lens, but this improvement is small, and it requires higher amount of phosphor, up to 25%, to achieve same CCT value.
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