The white light we see from LED is not a true white but a pseud white.
It is a fake white.
It is an invention of Nichia Kagku.
It uses a Stoke's Shift, or down conversion of light.
http://en.wikipedia.org/wiki/Stokes_shift
Stokes shift is the difference (in wavelength or frequency units) between positions of the band maxima of the absorption and emission spectra (fluorescence and Raman being two examples) of the same electronic transition. It is named after Irish physicist George G. Stokes.
When a system (be it a molecule or atom) absorbs a photon, it gains energy and enters an excited state. One way for the system to relax is to emit a photon, thus losing its energy (another method would be the loss of heat energy). When the emitted photon has less energy than the absorbed photon, this energy difference is the Stokes shift. If the emitted photon has more energy, the energy difference is called an anti-Stokes shift.
Well if the stoke's shift exist,
how about the other way around?
Yes, there is an anti'stokes shift.
Nature, God, is always kind to human.(・・。)ゞ