I. Introduction
Lighting is important for our daily lives, and electric lighting accounts for about 20% of total global electricity consumption. To decrease the power dissipation of electric lighting, light-emitting diode (LED) has become a popular solid-state light source because of its long lifetime, freeness of mercury, and high efficiency compared with other conventional lighting technologies [1]– [3]. It is predicted that LED lighting will occupy about 70% of the lighting market by 2020 [4].