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Re: [Phys-l] Interesting LED Light Effect- Reality Check Request



In my experience, the LEDs last forever.

But the circuit boards behind them burn resistors or something.

On Fri, Jun 24, 2011 at 11:31 AM, Roger Haar <haar@physics.arizona.edu>wrote:

Hi,
I think with CFL's some of the warm up is just getting the
mercury vaporized. Warm-up, outside, during the winter can
take more than a minute.

In both LED's and CFL's, if it takes seconds to get the
phosphors primed, it seems like it should take seconds to
for the intensity of the light to decay after the light is
turned off. I have observed a bit of this decay time with CFL's.

With LED's, I do not know what control circuits they might
have. They might monitor their own brightness, like some
diode lasers do. If this is the case, the feedback could be
setup to be slow, to provide some kind of protection for the
expensive LED.

Thanks
Roger Haar U of AZ

===================================================
On 6/24/2011 7:11 AM, chuck britton wrote:
Might this 'Heating the Phosphors' also explain why most CFL's need a
while to 'catch hold'?
Certainly sounds like a valid hypothesis.
.
At 8:59 AM -0400 6/24/11, Spinozalens@aol.com wrote:

I am in the process of replacing most of my incandescent lights with
LED
lighting. An interesting effect with these LED lights is that when they
first turn on they emit a somewhat purple colored light and are not
very
bright. However , after a short time the light shifts to what the eye
sees as
pure white light and they become as bright as the incandescent light
they
replaced. I am pretty sure I know what is going on here , but if
someone
knows better please make a comment. Here's what I think.

LEDs of course don't emit light over such a wide spectrum so as to
directly produce white light. You could combine blue, green and red
LEDs to create
white light but this is very expensive as the efficiencies of the
different color LEDs vary over a wide range and special circuits
would be needed to
adjust for this. A more cost effective method is to use UV or blue LEDs
and
broaden the light spectrum using phosphors.

What I am pretty sure is happening is that the phosphor needs time to
heat
up before it emits light over a broad spectrum, the purple tinge
reflects
the fact that the LED's being used are emitting UV light. I can't find
anything in the literature on LED lighting that talks about this,
hence my post.

Bob Zannelli
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l




--
Clarence Bennett
Oakland University
Dept. of Physics, (retired)
111 Hannah
Rochester MI 48309
248 370 3418