Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!bellcore!envy!karn
From: karn@envy.bellcore.com (Phil R. Karn)
Newsgroups: sci.electronics
Subject: Re: Do Halogen lamps need ballasts?
Message-ID: <1990Dec1.032426@envy.bellcore.com>
Date: 1 Dec 90 08:24:26 GMT
References: <FISH.90Nov28100429@his.gemed.com> <714@skipper.dfrf.nasa.gov> <1147@inews.intel.com>
Sender: usenet@bellcore.bellcore.com (Poster of News)
Reply-To: karn@thumper.bellcore.com
Distribution: sci.electronics
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Lines: 23

In article <1147@inews.intel.com>, bhoughto@cmdnfs.intel.com (Blair P.
Houghton) writes:
|> >Also, the quartz envelope shouldn't be touched by bare 
|> >hands -- oils on the skin will weaken the structure and may
|> >cause bubbles or lamp failure.
|> 
|> The oil causes a region of different thermal conductivity;
|> when the heat is applied this will cause a region of
|> different temperature which will cause a difference in
|> expansion which will cause a stress that will rupture the
|> envelope.  The oil doesn't "weaken the structure."  You can
|> wipe it off.

I don't think that's quite right. Quartz is used precisely because of
its
very low temperature expansion coefficient.

I think the problem is that the heat will decompose the oil, producing
elemental carbon which will then absorb the light hitting it from the
filament.  This causes a hot spot on the surface of the envelope which
could well cause it to soften and fail.

Phil


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