Re: Elementary Lecture - how fluorescent lights light

To: tesla-at-pupman-dot-com
Subject: Re: Elementary Lecture - how fluorescent lights light
From: Jim Lux <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Tue, 23 Feb 1999 18:30:06 -0700
Approved: twf-at-verinet-dot-com

> > Original Poster: James Mereness <"mereness"-at-dti-dot-net (Jamie Mereness)>
 "why does the fluorescent light glow", I'd
> > love to hear them.


Fluorescent tubes have mercury vapor inside them. Strong enough electric
fields rip the electrons off the atoms in the vapor (or move them to a
higher energy state, anyway). This takes energy.  When the electrons
return to their original position, they give back the energy in the form
of light (mostly invisible UV as it happens). The UV light hits the
white phosphor coating on the inside of the tube and causes it to glow
in visible spectrum.

Neon bulbs work the same way, except they use neon, which emits light
directly in the visible (red orange). Argon gives you a pretty violet
color. Xenon looks white.  Sodium vapor is bright yellow.

It matters not how the energy gets into the tube, whether by radiated
fields (a tesla coil), or electrodes actually in the tube (fluorescent
and neon tubes). The important difference from an incandescent bulb is
that the light does not come from heating up metal by running a current
through it.

Incandescent bulbs typically emit about 10% of the energy in visible,
the other 90% is IR (heat) (it is a black body radiator at about 2400K,
so the big peak is in the infrared). 

Fluorescent bulbs are much more efficient, perhaps 50%, by the time you
take the inefficiency of the ballast into account.  Xenon arc lamps are
probably another example of efficient lighting: lots of power goes into
making a xenon plasma, which happens to radiate with a spectrum much
like that of sunlight, so most of the power is in the visible.

Halogen bulbs are incandescent but, they run at much higher
temperatures, so they have better luminous efficiency (the black body
peak is closer to visible). Of course, the envelope has to be made of
quartz to keep from melting, and they have to put something in it to
keep the vaporized tungsten from depositing on the envelope (that's the
halogen, typically iodine))

-- 
Jim Lux                               Jet Propulsion Laboratory
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