OT: white LEDs

Mark Tapley mtapley at swri.edu
Tue Feb 23 17:51:01 CST 2010

At 12:00 -0600 2/23/10, Randy wrote:
>If you look at one in the microscope, it looks like an LED with some 
>sort of phosphor painted over the semiconductor.  Is that how these 
>things work, its really a high efficency IR LED, with a phosphor 
>doubler to bring the wavelength into visible?  Just speculating...

	There are a few crystals that can double frequency (halve 
wavelength, thereby *increasing* the energy per photon). I think 
these are used in laser ranging, to bring a powerful IR laser beam up 
into the visible so it'll transmit better through the atmosphere.
	I think no phosphors can do that. They can bring frequency 
*down* (ie absorb an ultraviolet photon, emit 2 visible or one 
visible and one IR or some combination). In fact, I don't recall ever 
hearing of a device that can do that with non-coherent light (well, 
as direct conversion). The problem is trying to gather 2 low-energy 
photons into a single high-energy photon. That turns out to be tough 
to do in general.
	There are phosphors that can be "pumped" by blue or UV light, 
then stimulated to re-emit visible light when IR photons hit them. (I 
think the Germans made IR goggles this way in WWII.)

						- Mark     210-379-4635
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