LED of various light colors was widely used in traffic signal lights and large-area display screens, which produced good economic and social benefits. The 12-inch red traffic light, for example, was originally used in the United States as a long-life, low-light-efficiency 140-watt incandescent light source, which produces 2,000 lumens of white light. After passing through the red filter, 90% of the light is lost, leaving only 200 lumens of red light. For general lighting, people need white light source more. In 1989, the white LED was developed successfully. This LED is made of GaN chip and yttrium aluminum garnet (YAG) encapsulated together. GaN chips emit blue light (λ P =465nm, WD =30nm), and the Ce3+ containing YAG phosphor made by sintering at high temperature is excited by the blue light and emits yellow light with a peak value of 550nm.

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