Thats an interesting application, makes sense though.
To add to that, an important difference apart from the pure temp, is that
because the insulators are obviously a hell of a lot more dense than air,
for a given temperature they will produce significantly more radiation to be
picked up by a camera. Unfortunately, it's the worst case scenario detecting
thermals in the air - a substance with low density, and low heat
differential at moderate to long distances.
Luke
On 27 August 2010 23:22, ANDREW BARDGETT <[email protected]>wrote:
> 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]<http://uk.mc865.mail.yahoo.com/mc/[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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