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From: arno@utu.fi (Arno Hahma)
Subject: Re: Anti-thermal Fog
Date: Tue, 30 May 1995 23:03:59 GMT

In article <D95vL4.BAB@ranger.daytonoh.attgis.com>,
Norman Purves <purves@cfht.hawaii.edu> wrote:

>>armed forces (US, I think) had developed a chemical fogging agent that
>>was able to blur the thermal signatures of tanks and APC's under

Such smokes do exist. They are very effective, even in pretty low
concentrations they obscure a tank or similar against UV, VIS and 
IR detection enough for a missile to miss. In addition, IR-smokes are
more effective than conventional ones in general.

>No refs, other than a physics-textbook. If the smoke is hot, e.g. white
>phosphorus, each individual particle will emit thermal radiation, which
>will tend to mask other thermal sources within and behind the smoke-cloud.

It works only for a very short time. The smoke cools down quickly
(within seconds) and you are back to absorption only.  Phosphorus smoke
has a little drawback, too. While it is very effectively screening
in visible wavelengths, it has a completely clear "window" down
in the IR area.  If you tune your IR-designator to that "window", the
phosphorus smoke has no effect. Certain additives in the smoke can
block this window, though.

>If smoke can be produced with carefully controlled particle-sizes, in this
>case about 10 microns (where most thermal-imaging systems work) the effect

This approach is used widely today; 10..15 year old technology now. The
smokes are essentially dusts, that are scattered into the air by
pressurized gas or explosive charge.  They work great, but
unfortunately, they stay only a short time in the air. Most present
IR-smoke systems in vehicles use dusts.

FYI, there is a very nice, IR-smoke dispenser available nearly
everywhere:  fire extinguishers. The powder-extinguishers make up an
excellent IR-smoke, they obscure the target completely against even a
far-IR camera. Blast one into the air and the IR-whatever won't see
through the cloud; el-cheapo and effective.

>Norman Purves
>Canada-France-Hawaii Telescope
>Mauna Kea, Hawaii

ArNO
    2

 
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