From:	Marc Renouf <renouf@********.COM>
Subject:	Re: thermographic vision-question
Date:	Tue, 2 Feb 1999 15:16:00 -0500

On Tue, 2 Feb 1999, Jester wrote:

> Could someone please post a mail explaining what thermographic vision
> is, and how it works. Preferably long, with lot's of difficult words
> in it (his English isn't that good :) ).

Okay, here's how it works: Infrared radiation behaves pretty much
like any other form of electromagnetic radiation. It is directly
analogous to light or radio or microwaves.
Typically, the infrared band is considered to be more or less in
the 3 to 20 micron range (i.e. the wavelength of the waves as they
propagate is 3-20 microns long). Whether or not a substance is
transparent to radiation of a given wavelength depends largely on the
size of the crystal structure of the object in question. Most objects
(wood, concrete, rock, water, and even some types of glass) are more or
less opaque to radiation of this wavelength. As such, you won't be able
to "see through" these kinds of materials when using thermographic vision.
However, having said that, I will point out that thermal energy is
transferred around by a variety of different mechanisms, of which
radiation is only one. Heat is also transferred directly via conduction.
That it, if two objects are touching, heat will move from the hot one to
the cool one. This is how an electric stove works. The water is in
contact with the pan, and the pan is in contact with the heating element.
Heat moves from the hot heating element to the pan, and then to the water.
It doesn't radiate through the pan directly to the water (although that
would be kind of cool).
This phenomenon can be used to one's advantage however. As
something cools, the heat needs to go somewhere (a la conservation of
energy). If something warm sits in one place for a while, its heat will
bleed off into the surrounding environment as it cools. This raises the
ambient temperature of things around it. Something that generates its
own heat (like a warm-blooded mammal, perhaps), it will continue to heat
the things around it. For example, if you sit in a chair for a while and
then get up, the chair is still warm, because it has been heated by your
body.
How does this help you? If a security guard has been leaning up
against the same wall for a half hour, his body heat will have "bled
through" the wall to some extent, simply because his body is heating the
wall via conduction. As an object heats up, it will radiate or conduct
the heat away just like the thing that heated it. So the wall will
conduct heat to the floor and ceiling, but more importantly, it will
radiate thermal energy in this same 3-20 micron wavelength band. It is
this energy that your thermographic vision sees. So when the guard leans
against the wall for a half hour, he creates a warm spot that you can see
on the other side.
How long does it take this heat signature to propagate through a
material? It depends on the thermal conductivity of the material. Some
materials (like metal) conduct heat very quickly (which is why they make
good pots and pans). Others, like fiberglass, conduct heat very slowly
(which is why they make good thermal insulation for your house). Also,
you need to consider the mass of the object. Density, thermal
conductivity, and thermal capacitance (how much heat a material can
"hold") are very important to determining how long it takes to be able to
see someone's heat signature through another object. Obviously 3 feet of
concrete warms up an awful lot slower than a 1" piece of steel plate.
Basically, the longer something stays in one spot, the easier it
will be to see. Leave it up to GM discretion as to how long it will take.
But even if the person's heat signature bleeds through an object,
it may still be a fuzzy, indistinct blob, especially if the barrier is
very thick or has poor thermal conductivity. As such, I generally apply
some modifier (+2 to +4) to shots through barriers or whatever, as you
can't quite tell where a person is and just sort of aim for the middle of
the blob.

I hope this answers your question.

Marc