Re: Gas discharge lamps was Re: Blue Bulb Physics

[Robert Myers <rmyers@inetone.net>]

Marty,

Here I go again, picking at nits.

The halogen functions to react with tungsten vapors which are expelled from
the filament by the extreme heat.  Ordinarily, these tungsten vapors would
be deposited on the comparatively cooler glass envelope.  However, the
tungsten vapors react with a small amount of iodine (or bromine depending
on the manufacturer) and forms WI3 (or WBr3).  

2 W + 3 I2 <==>  2 WI3

The WI3 is quite volatile at the operating temperature of the bulb and a
significant portion if not most remains in the vapor state.  When this
vaporized WI3 contacts the *extremely* hot filament, the WI3 decomposes
back to W and I2.  The tungsten is deposited back on to the filament
surface, thus replacing the tungsten which had vaporized at first.  The I2
is then free to go back and, so to speak, pick up another load of tungsten
to deposit on the filament.  This total process tends to keep the tungsten
where it belongs (part of the filament) rather than on glass surfaces,
etc., and results in a significantly longer bulb life compared to an
"ordinary" incandescent light without the halogen.  What's more, the
scavenging effect of the halogen keeps the transparency of the envelope
much better since nearly black metallic tungsten deposits are removed from
the glass.

The higher pressure inside the bulb has little to do with its longevity.
It is the halogen which allows the increased current and the brighter
lighting because it can provide a much greater longevity that the bulb
would have without the halogen.

At 08:53 AM 4/17/98 -0600, you wrote:
>A recent project I worked on involved using gas discharge lamps because of
>their emissions in the blue frequency range and the fact that blue lasers
>are very, very expensive.
>
>These lamps are very efficient.  The majority of the input energy is
>converted to photons, unlike normal incandesent lamps.  When we first got
>the lamps (50 Watts) with a lens that puts the majority of the light into a
>relatively parallel beam, we turned it on and with in a very few minutes
>the wall, about 8 feet away, was smoking.
>
>Unfortunately for us, the lamps didn't provide enough blue light.  A 125
>Watt lamp only gave us about 8 milli-watts of blue light, so we ended up
>using a laser anyway.
>
>BTW, halogen lamps are just incandesent lamps with the filament encased in
>a small glass bulb containing high pressure inert gas.  The high pressure
>gas keeps the filament intact under high current conditions.  It's the high
>current that makes halogen lamps brighter than equivalent normal
>incandesent lamps.
>
>Peace
>Marty Halvorson
>
>
>Peace
>
>Marty Halvorson
>Judicial Information Division
>NM Administrative Office of the Courts
>martyh@nm.net
>
>
>
>
___
   Bob
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