On Aug 19, 2005, at 11:36 AM, Debbie wrote:

> PICers - just curious about "White LEDs" that you see in the
> catalogues. Are they truly broad spectrum white-light, as in
> daylight or from a fluoro tube?

There are several varieties of white LEDs with different specta.
I think the most common is a Blue LED with basically yellowish
phosphor, yielding a rather double-peaked spectrum.  Realizing
that this was less than ideal, some manufacturers are using
multiple phosphors to even out the spectum a bit, with either blue
or UV led chips  (VERY much like cheap fluorescents vs "daylight"
fluorescents, in fact.)  Still other LEDs use red, green, and blue
emitters...  You need to view spec sheets for the details (which
of course can be a problem if you were planning on taking advantage
of the inexpensive chinese white LEDs available from eBay.)

Exact spectra also seem to be a significant quality control issue.
Lumiled Luxeon LEDs are sorted by the exact color/spectra they
end up producing (as well as actual brightness and Vf behavior.)
(by "Coordinated Color Temperature", which ranges from 5350 to
8000k - quite a range, and well large enough to see the differences
and have least-favorite bins.)  As other people have mentioned,
beam color can vary (especially from center to edges) even within
a single LED (although this seems to be less common recently.)

BillW
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist