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.)
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