From:	Marc Renouf <renouf@********.COM>
Subject:	Re: thermographic vision-question
Date:	Wed, 3 Feb 1999 16:45:13 -0500

On Wed, 3 Feb 1999, David Woods wrote:

> Well, it a difference of degree. Clothes will be warmer than ambient,
> but by how much? 2-3, 10 degrees?

It really depends on the type of clothes and the ambient
temperature. A light shirt will be a lot warmer externally than a parka
that insulates well. Systems even today can distinguish fractions of a
degree.

> Assuming your only wearing a shirt. If your wearing armour it's not
> going to be far above ambient.

It doesn't matter. You're still in physical contact with it, and
thus still talking about conductive heat transfer.

> Yes, but what if it's about 12C with a stiff wind and your wearing
> insulating clothes plus a flak jacket. You lean up against a brick wall.
> How long would it take to see noticable heating of the oposite side?
> Would it ever, or would the large heat capacity and rapid conduction
> along the wall render it beneath delectability?

It very well may. However, the colder the ambient temperature is,
the more likely you are to be able to see someone with thermo. It's not
in the arctic that you have problems, it's in the jungle. Conductive heat
transfer is proportional to the difference in temperature of the objects
in question. The greater the difference, the faster the cold one will
heat up and the hot one will cool down. In a situation where you have an
extreme gradient, changes in temperature will happen quickly.

> I'm not convinced it would ever be seconds. But I might be wrong. GM's
> call really.

Like I said, it really depends on the type and thickness of the
material in question. But for thin materials with a high thermal
conductivity, seconds is certainly within reason.
Also, keep in mind what that means. It means that you know that
the wall is warmer. It doesn't mean you know what's behind the wall or
what position the person is standing in. You may be able to see a
person's general location long before you'll have enough information to
get off an accurate shot.

Marc