Archived posting to the Leica Users Group, 1996/08/26
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From: Wolfgang Sachse <sachse@msc.cornell.edu>
Date: Sun, 25 Aug 1996 09:29:30 -0400 (EDT)
The only way that could be true is if the fan is causing
excessive vibrations of the (unlit) filament. If this
projector causes that, it shows poor engineering. Lousy
design of the fan, its mounting, the mounting of the bulb
and fragility of the bulb itself.
Given that we're all shooting in the dark here, I don't see why you're
saying "The only way that could be true..." -- certainly I can come up
with a plausible-sounding explanation. Suppose that using the fan on
the extinguished bulb causes the surface of the bulb to cool more
quickly than the glass immediately under the surface. Then thermal
stress might create very small cracks in the glass. If small amounts
of air seep into the bulb through these very small cracks, the
filament will oxidize and fail sooner than if it had been kept in a
better vaccuum.
I don't think that this is necessarily the mechanism by which cooling
an extinguished projector bulb with a fan causes it to fail sooner, or
even that there necessarily is such an effect -- but your flat denial
of the existence of this phenomenon without presenting any evidence
strikes me as a little bit too quick.