Le dimanche 2 octobre 2016 20:57:45 UTC+2, Jonathan St. Cloud a écritÂ*:
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 monocle. Not sure which sensor array was picking that up, but I thought it was the FLIR.

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 wrote:
A FLIR unit might offer a visual indication from much larger distance?

A few moons ago, I got chatting to a guy selling FLIR imaging systems at a trade show. I asked him about the problem of trying to see thermals, so we set an imager up to look at the hot air rising from a nearby vent and saw nothing (as he expected).
There's no problem seeing warm solid objects, since they consist of a lot of closely-packed warm emitters - high spatial density - but warm gas is so much more diffuse that any infrared 'brightness' just fades into the background, like a small amount of dye in a large volume of liquid. To do effective 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 the best thermal indicators, if they can be bothered to fly where we want them to.

DH
TX

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

Hot air emits infrard radiation. However, as emission and absorption coefficients 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. That's how physics works. The stories about people having seen infrared images of thermals are just urband legends.

Bert (who has been developing infrared sensors and systems for more than 15 years)