On 25/04/2017 14:54, James Wilkinson Sword wrote:
On Tue, 25 Apr 2017 14:27:29 +0100, Martin Brown
wrote:

On 25/04/2017 13:27, Graham. wrote:
On Tue, 25 Apr 2017 12:25:35 +0100, "James Wilkinson Sword"
wrote:

Yes, I've googled it, but there are many different approaches.
Which one is used in domestic LED bulbs?

Ultra-violet LED exciting a white phosphor, much the same principle
used in florescents.

It is usually a high efficiency blue LED pump exciting a yellow phosphor
with that mix determining the nominal colour temperature. It is quite
peaky in the blue and more of a wide hump around the yellow. The
phosphor usually looks yellow and sits on top of the LED.

http://i.stack.imgur.com/lkyXG.png

That's not as even as I thought. I thought they'd made it similar to
the sun, which looks like this:
https://i.publiclab.org/system/image...ctrumGraph.jpg

Maybe we should be using the controllable colour RGB ones.

They would be even worse at approximating the solar spectrum.

https://s-media-cache-ak0.pinimg.com...07cc796e89.jpg

RGB LED emissions have a fwhm of 50nm so would have a very peaky
spectrum. It doesn't normally matter except for colour matching or where
you have unusually narrow band pigments.

Materials that have very different colour depending on the white light
source you use are called after the semiprecious stone Alexandrite.

https://en.wikipedia.org/wiki/Chrysoberyl#Alexandrite

The other common one is neodymium doped glass used by glassblowers to
see into a gas flame against the yellow-orange glare of sodium emission.

--
Regards,
Martin Brown