Archived posting to the Leica Users Group, 1998/03/12

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Subject: Re: [Leica] Off-Topic: Airport X-ray Machines, Physics 101
From: ATanabe <ATanabe@aol.com>
Date: Fri, 13 Mar 1998 00:24:58 EST

As a rule the thicker the object (lead bags) the more energy (Xray) needed to
penetrate the object.  For example, xray penetration or energy is measured in
KiloVolts (KV) the higher the energy the greater the interactions with objects
in its way (bouncing of electrons) = fogging of film.  With the emulsions
today, there is less silver than old emulsions, silver being metalic, reflects
the xray energy and causes the halides to be fogged.  One can duplicate this
scatter by piling on cassette after cassette on one another (metal casings
will cause the scatter).  The best way to survive airport xrays (if you have
to) is to have a single layer of cassettes on the belt, making the energy
lower (less mass) and reducing the scatter.  If you put the film in a lead
bag, the machine will automatically crank up the energy (auto exposure) on the
xray and fog your film.  
Al