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

[Roger Haar <haar@physics.arizona.edu>]

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
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