IR cameras are used to detect 'hot spots' on electricty grid insulators. This
is normally carried out in the air from a helicopter and the pictures I have
seen show plenty of contrast against the beckground. The problem with a thermal
is that there is a large quantity or air not many degrees above ambient and
with its edges mixing with the ambient air. Most other subjects, such as
insulators or human beings are at at much higher temperature than ambient and
have a sharp edge !
I suspect the technology is still too large and power hungry for applications
in gliders just yet.
Andy
--- On Fri, 27/8/10, Luke O'Donnell <[email protected]> wrote:
From: Luke O'Donnell <[email protected]>
Subject: Re: [Xcsoar-user] Fwd: Infrared and thermals
To: [email protected]
Date: Friday, 27 August, 2010, 12:45
Valid points. I suspect that you wouldn't be necessarily looking for just hot
spots, but perhaps looking at rapid decreases in temperature. Depending on the
level of achievable contrast as well as how muchj temp trace there is when a
thermal moves though, it might be possible to visually 'see' the thermal
tracking over the ground, cooling it as it goes. Still, i would be suprised if
it would be nearly as easy to use IR to detect changes in temperature of air,
as opposed to changes in temperature of the ground.
I'm not an expert in this topic by any stretch of the imagination, so i could
be completely off the mark. However, a read of:
http://en.wikipedia.org/wiki/Thermal_conductivity
Indicates that there is a close relationship between thermal conductivity
(absorption of heat) and transmittance (losing heat, such as through IR
radiation). Some figures are (lower = less conductivity/transmittance):
Air 0.025
Wood 0.04 - 0.4
Water (liquid) 0.6
Soil 1.5
Concrete, stone 1.7
So soil has a conductivity around 60 times that of air. I suppose it follows
reason that as such, it would (per unit) output substantially more amount of IR
radiation than air. As such, detecting it with a camera should be much easier
than detecting changes in air. The other problem with attempting to directly
measure the temperature of air at a distance with IR, is that because of the
relatively high amount of IR being produced 'behind' the thermal, it would
likely totally whitewash any radiation you might pickup from the thermal. I
suppose there is an argument along the lines that if you have the camera
pointed above the horizon, you would not be viewing this radiation. However i'd
be surprised if this turned out to be practical - any form of cloud and perhaps
even just high humidity would probably drown it out at the sorts of distances
that would be useful ( a few km). Unfortunately radiation has a nasty habit of
decreasing at square the distance.
It's interesting stuff anyway.
Luke
On 27 August 2010 22:00, Martin Gregorie <[email protected]> wrote:
On Fri, 2010-08-27 at 20:52 +1000, Luke O'Donnell wrote:
> I wonder if a more feasible approach to thermal detection is using IR
> cameras pointed at the ground. From what i can gather, air is a poor
> medium for producing IR radiation, whereas presumably any solid object
> (anything on the ground) would be much, much easier to detect.
>
I think the problem is not to find hot spots that might act as thermal
generators, but to know where the thermal is. For example, a flat black
concrete roof or car park is going be be bright in IR on a sunny day,
but there is unlikely to be a continuous standing thermal above it.
Similarly, a thermal travelling across a hayfield will certainly be fed
by hot air pooling among or just above the grass stems, but it probably
will have little effect on the temperature of the dry grass it crosses,
so how can your camera see where the thermal is in a large field or even
if there is one?
> A real life example is the police helicopters following suspects at
> night with ease, they light up like a Christmas tree.
>
They also have quite a large temperature difference to work with. The
back of my hand is currently 28.5C in a room at 21.3C, a 6.2C
difference. Here its a cold, damp overcast day for summer and my heating
is off. Outdoors at night the ground temp is going to be around 10-15C,
and a moving perp is unlikely to be colder than my hand, so we're
looking at a 13-18C temp difference for the camera to spot. The air temp
of a thermal is somewhere between 0.2-2.0C above the surrounding air, so
I suspect its impossible to pick out against the natural variations in
daytime ground temperature: it would be hard enough to spot against the
background air temperature looking horizontally.
I'd well believe that an IR camera can see the hot air column above a
candle flame with the ambient room temperature as background, but again
that's a big temperature difference. You don't leave your hand above a
candle flame for long!
Martin
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