light can be made different ways - by mixing reds, greens, and blues,
by using an ultraviolet LED to stimulate a white phosphor (the same
stuff that's inside a fluorescent bulb), or by using a blue-emitting
diode that excites a yellow-emitting phosphor embedded in the epoxy
dome. The combination of blue and yellow makes a white-emitting LED.
Combine a white phosphor LED with a few amber ones, and you can create
a range of different whites - from the romantic glow of a candle flame
to the hot, bright light of the sun.
Most "white" LEDs in production today use a 450nm – 470nm
blue GaN (gallium nitride) LED covered by a yellowish phosphor coating
usually made of cerium doped yttrium aluminium garnet (YAG:Ce) crystals
which have been powdered and bound in a type of viscous adhesive.
The LED chip emits blue light, part of which is converted to yellow
by the YAG:Ce. The single crystal form of YAG:Ce is actually considered
a scintillator rather than a phosphor. Since yellow light stimulates
the red and green receptors of the eye, the resulting 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 blue emitting phosphors plus green emitting copper and aluminium
doped zinc sulfide (ZnS:Cu,Al).