"William Sommerwerck" wrote in message
...
Is there really such a thing as a white LED? The ones I have seen have
all
been red/green/blue LEDS on the same substrate to produce what appears to
the eye as a white beam, most of which are far too blue for my taste.

Have you never seen the ones that use a blue LED and a yellow-fluorescent
pigment?


They are blue because blue LEDs have a much shorter life than red and
green
so the color will change as they age, and they start out blue before the
end up
a red green mix (yellow/orange).

What?

I have never seen a dead LED (though I assume they exist), nor have I
heard
of LEDs becoming dimmer with age.


You're not quite correct there. They do dim with age, and that is actually
the way that they are specified for lifetime expectancy. I seem to remember
that it is something like 'hours to the 50% point'. The figure drops
drastically if they are DC driven rather than pulse driven, and if they are
'abused' with excess current. I have also seen dead LEDs in indicators,
bargraph displays, and where they are used as some kind of voltage reference
in amplifier output stages.

Arfa