From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Newsgroups: sci.chem,sci.engr.lighting
Subject: Re: Fatty Fingers and Halogen Bulbs
Date: Sat, 09 May 1998 09:50:34 GMT

mtp@mindspring.com (Matthias Pohl) wrote:

>Quite a while back (> 9 mo.?) there was a post in this
>group about why not to touch halogen bulbs with bare
>hands.  Anybody has a copy of this old post or can point
>me to some original literature about this subject.

I've added sci.engr.lighting to the newsgroups line, as
sci.chem isn't really the place for this. The historical
reason for not touching any bulbs was that our fingers
have a diverse layer of lipids, salts, and other material
on the surface.

Some of this gunge will be deposited on the lamp surface
and as the lamp is operated the heat converts the gunge
to an insulating film that decreases the heat transfer
rate and that part of the bulb can overheat. With modern
lamps of high output and colour temperature ratings,
they operate at temperatures sufficiently high to possibly
char the organic material into carbon - which is thermally
insulating, creating thermal stresses that can lead to
premature failure of the bulb.

The sodium present in the gunge salts could initiate
devitrification of quartz at temperatures around 900-1000C.
I'm not certain which of the two mechanisms ( surface film,
devitrification ) is the most likely cause of failure for
quartz-halogen bulbs - but most instrument manuals
emphasize that all traces of gunge should be carefully wiped
from bulbs prior to use. Perhaps the experts can enlighten
us all. Sorry, I've no references.

        Bruce Hamilton