“…The disadvantage of the conventional lamp is short life time (~1,000 hours), complex optical device is needed, high power consumption, low luminous efficiency, and high maintenance cost. [1] On the other hand, LEDs have more specific advantage, such as high luminous efficiency, low operating voltage, low power consumption, long life time (~5,000 to 10,000 hours) and absence of complex optical devices. Moreover, the LEDs have specific light spectral, and is easier to control light intensity.…”