Remote thermal detection

At 13:03 03 October 2016, Tango Whisky wrote:
Le dimanche 2 octobre 2016 20:57:45 UTC+2, Jonathan St. Cloud
a =C3=A9crit=
=C2=A0:
In the eighty's or nineties there was an article in "Soaring" by
an U.S.
=
Army Apache trainee, who noticed he could see raising thermal
through his
m=
onocle. Not sure which sensor array was picking that up, but I
thought it
=
was the FLIR.
=20
On Sunday, October 2, 2016 at 11:04:32 AM UTC-7, David
Hirst wrote:
On Monday, October 3, 2016 at 4:04:39 AM UTC+13,
Jonathan St. Cloud
wro=
te:
A FLIR unit might offer a visual indication from much larger
distance=
? =20

A few moons ago, I got chatting to a guy selling FLIR imaging
systems
a=
t a trade show. I asked him about the problem of trying to see
thermals,
s=
o we set an imager up to look at the hot air rising from a nearby
vent and
=
saw nothing (as he expected). =20
There's no problem seeing warm solid objects, since they
consist of a
l=
ot of closely-packed warm emitters - high spatial density - but
warm gas
is=
so much more diffuse that any infrared 'brightness' just fades
into the
ba=
ckground, like a small amount of dye in a large volume of liquid.
To do
ef=
fective background subtraction, you need to know what the
background is to
=
begin with and on a typical thermal day this is the average
temperature of
=
the air which has high spatial variability.
I think birds can see all those rising insects, which makes the
birds
t=
he best thermal indicators, if they can be bothered to fly where we
want
th=
em to.
=20
DH
TX

You cannot detect hot air from a distance by any infrared detector
(and
FLI=
R is infrared imaging). Obviously, your sales guy wasn't up to
speed with
p=
hysics.

Hot air emits infrard radiation. However, as emission and
absorption
coeffi=
cients are the same thing, the air inbetween the thermal and your
FLIR
will=
absorb all of this radiation, and you won't see anything on your
imager.
T=
hat's how physics works. The stories about people having seen
infrared
imag=
es of thermals are just urband legends.

Bert (who has been developing infrared sensors and systems for
more than
15=
years)
In the early 70's Wally Scott was flying a OD Green 1-34 ? for MIT
and they were trying to see dust particles in thermals in west texas.
with todays tech. it may be possible.any body else remember this.