From: John Beckett 

"Phil Payne"  wrote in
message news::
> "John Beckett"  wrote in message
> news:v52ku2hvqrv67f5mdvka1bf878r6rrdn45{at}4ax.com...
>
> > I think that the light emitted by a LED is in a very narrow band of
> > frequency (at least it was, when I learnt about LEDs).
So a red LED
> > emits light (almost) only at the red frequencies.
> >
> > Therefore, you can't just filter it to whatever colour you want.
>
> And a white LED?

There may have been one or two developments since I last learnt about
LEDs, but as I understand it my statement above is correct because
I'm specifically referring to a red LED (one of the bright and cheap
colours).

I believe a white LED is achieved by having some kind of chemical absorb
whatever narrow-band frequency is emitted by the LED, them re-emit in a
broad spectrum, i.e. white.

So yes, you could filter a white LED to produce any colour, but I suspect
it wouldn't be very efficient.

It's a trivial point, but Geo was saying that LED garden lights are only
yellow because of a yellow plastic case. I doubt that.

John

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