1. IntroductionDuring the last four

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.