White light can be made different ways - by mixing reds, greens, andblues, by using an ultraviolet LED to stimulate a white phosphor (the samestuff that's inside a fluorescent bulb), or by using a blue-emitting diode thatexcites a yellow-emitting phosphor embedded in the epoxy dome. The combinationof blue and yellow makes a white-emitting LED. Combine a white phosphor LEDwith a few amber ones, and you can create a range of different whites - fromthe romantic glow of a candle flame to the hot, bright light of the sun.
Most "white" LEDs in production today use a 450nm – 470nm blueGaN (gallium nitride) LED covered by a yellowish phosphor coating usually madeof cerium doped yttrium aluminium garnet (YAG:Ce) crystals which have beenpowdered and bound in a type of viscous adhesive. The LED chip emits bluelight, part of which is converted to yellow by the YAG:Ce. The single crystalform of YAG:Ce is actually considered a scintillator rather than a phosphor.Since yellow light stimulates the red and green receptors of the eye, theresulting mix of blue and yellow light gives the appearance of white.
White LEDs can also be made by coating near ultraviolet (NUV)emitting LEDs with a mixture of high efficiency europium based red and blueemitting phosphors plus green emitting copper and aluminum doped zinc sulfide(ZnS:Cu,Al).
