1. Introduction
During the last four decades, the technical progress in the development of light-emitting diodes (LEDs) has been breathtaking. State-of-the-art LEDs emitting in the vis-ible spectrum are small, rugged, reliable, bright, and highly power efficient. Nowadays, illumination LEDs are predicted to be used in myriads of applications [1]. Since
LED illumination inherits the advantages of standard LEDs, it offers numerous advantages, such as a consid-erable power reduction, extreme durability, size reduction, a greater degree of freedom for illumination shape, and a high resistance to shock and vibration. In addition, LED-based lighting requires less maintenance costs due to its long lifetime, and the chip per unit price of luminous flux is continuously decreasing. Due to these character-istics, LEDs have become very attractive, not only for illumination but also for outdoor and indoor decoration and advertisement.