There are several ways using LEDs to produce white light. The first one uses three or more monochromatic color lights, such as blue light, red light, and green light, to produce white light. Another way is mixing two complementary color lights, such as blue light and yellow light. The blue light is usually generated by a nitride light-emitting diode; yellow light is generated by exciting phosphor through blue light. The white light generated by two complementary color lights generally has a better luminous efficiency but worse color rendering index than that generated by three monochromatic color lights.
Color rendering index is a measure of the ability of a light source to render the true color of an object illuminated with the light source in comparison with daylight. A light source with a higher color rendering index can render more realistic color of an object. The Halogen lamp and the incandescent bulb have better color rendering indices, which can reach to 100, among the artificial light sources. The fluorescent light has a color rendering index of about 60˜85. The white light generated by the blue light-emitting diode and the yellow phosphor merely has a color rendering index of about 70. Although two or more phosphors, such as yellow and red phosphors, can be placed on the blue light-emitting diode to increase the color rendering index up to about 80, the luminous efficiency is decreased by 30%.