From:	Marc Renouf <renouf@********.COM>
Subject:	Re: thermographic vision-question
Date:	Wed, 3 Feb 1999 14:21:03 -0500

On Wed, 3 Feb 1999, David Woods wrote:

> The surface temperature of a human is very dependent on ambient
> temperature. In the average run most people will be wearing clothes :).
> So the surface in contact with the wall will be fairly close to ambient,
> not blood temp.

Not so, my good man. Your clothes are typically warmer that the
surrounding environment. Easy example? Grab a shirt out of your closet.
Now take the shirt you're wearing off. Hand them to someone. They will
be able to tell you (without even having to smell them! ;) ) which one
you were wearing, simply because that shirt will be warmer than the
other (which is at ambient temperature because it's been sitting in your
closet).
Further, if you're leaning up against a wall, there is no air flow
between you and your shirt, or between the shirt and the wall. This
airflow is necessary for convective heat transfer (heretofore
undiscussed). Convective heat transfer is what keeps your clothes cooler
than your body. In the absence of such convection, your shirt will
rapidly heat up to your body temperature, and will rapidly begin heating
up the wall. Easy example? Driving your car on a hot summer day. Even
if the rest of you is cool and dry, your back gets all sweaty because
there's no cooling convective airflow between you and your seat.

> Imo very little heat would be transferred to the wall. It would be tens
> of minutes before a noticeable change was seen.

Again, it depends on the wall. In some cases, you might be able
to discern the change in just a few seconds. You may not be able to tell
whether the guy hiding behind the wall is an elf or a human, but you know
there's a guy hiding behind the wall.

Marc