How do produce white LED’s and what are its existing problems? *?
Answer:
Previous posts have it partly right -- White light is achieved in three different ways.
1) The first method is color mixing, using individual LED colors: blue and yellow to create a dichromatic white source; red, blue and green LEDs to create a trichromatic white source and blue cyan, green and red LEDs create a tetrachromatic white source.
2) A second method is binary complementary wavelength conversion. A blue LED is complemented with a yellow phosphor to create cool white light with a typical color temperature of 5,500K. By adding a secondary red phosphor, a warm white color with a color temperature of 3,200K is achieved.
3) The third method is ultraviolet wavelength conversion, in which a single ultraviolet LED is used to excite a tri-color phosphor coating, which in turn produces visible “white” light.
The biggest existing problem, depending upon which of the 3 construction methods above is being used, is color shift -- they don't necessarily stay "white":
All of the white LED products have deficiencies, but the manufacturers are constantly working on these shortcomings. For example, all “white” LEDs suffer from a color shift over time. Also, color mixing using various color LEDs to create a combination that emits a white light has quality issues over time, because different materials degrade at different rates. In addition, the manufacturing methods used may also influence depreciation in the same basic color.
One of the biggest misconceptions about LEDs is that they are cool light sources, because the LED doesn't generate infrared energy. But, unlike an incandescent bulb, which sheds its heat radiatively, an LED must dispel heat by convection and conduction. If the LED die junction does not operate below its maximum rated temperature during operation, both light output and life are reduced.
While LEDs are considered to be reliable and to offer long life, the lighting industry has no agreement on the definition of an LED source or the useful life of the LED. One possible definition of “lifetime” is the number of hours required for the source output to decline to a certain percentage of its initial output. Many specification-grade LEDs achieve 70 percent lumen maintenance after 50,000 hours of use under standard operating conditions.
to make a white LED, they build a blue LED with a phosphor coating. the conversion from the phosphor makes it inefficient
You need a red, a green and a blue LED to make white light. The hard part is making the blue one; that wasn't done even experimentally until 1992. Right now, they're just starting to sell cheap reliable blue LEDs, so soon we'll see white LEDs and full-colour LED displays in consumer products.
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