Does the shimmering effect of the desert heat affect thermal imaging systems? Are they able to see through the “mirage” or does the image waver like it does to the naked eye?
My only experience with thermal imaging cameras has been lowish-resolution cameras (640x480 was the best, most were in the 320x240 ballpark), but I have had a ton and a half of experience with them (every day I’ve gone to work for eight years). I have not seen the heat wave shimmer looking across nearly two miles of hot airport on a summer day, and I have looked. There is plenty of shimmer visble to the naked eye at the times I’ve looked for it, but nothing has shown up on the screen.
I would suggest that the problem lies with the display screen being so small, and the refresh rate of the cameras being slow enough (in the 20-30 hz range) that you could not make out any shimmer. I don’t know if that is the exact reason, but I can assure you that the cameras we have are not super-duper military cameras with every available technological upgrade. The cameras are in the $10k to $20k range, though, so they are pretty good. The cameras are also optimized for viewing in the 50’ to 200’ range, not miles away, so that might have something to do with it also.
If you have a better camera, perhaps you might see the shimmer; but with the general consumer- and fire service-grade cameras, there is no shimmer visible.
We use FLIRs at work, I’ll ask the guys tomorrow if they notice heat shimmer. I used to operate them myself and I don’t recall noticing heat shimmer but then I wasn’t particularly looking for it. The fact that 99.999% of the time we use over the ocean doesn’t help, you don’t get heat shimmer out there, but we test them each day on tarmac where heat shimmer might be more obvious. I would be surprised if they didn’t pick up heat shimmer, if there’s enough convective movement of the air to bend light I would’ve thought it would effect the lower wave lengths just as much if not more.
I think the shimmer is refraction, caused by different velocities of light within different temperatures (and so densities) of air.
I don’t know the equation, but I would guess that longer wavelengths refract less because when you split white light using a prism, it’s the red side that is nearest to a straight line through the prism, and the violet side that bends most.
So, infrared is longer wavelength than visible light = less refraction = less heat shimmer
Maybe
Yes good point.
The reason I asked is that in an account of desert fighting by a former Afrika Korps general, he related how the severe heat shimmer could obscure enemy forces. I wonder if our current desert fighters have had to deal with this or if thermal imaging sights on M-1s can cut through the shimmer.
Interesting thought. I’ve always remembered which rays bend the most by reasoning that a magnetic field is light with a frequency of zero, and a glass prism doesn’t bend magnetic field lines. Though, this has always been merely a way of remembering, and I don’t actually think it’s reasonable optics.
The property of transparent substances that makes different wavelengths of light bend differently is called the index of dispersion (not to be confused with index of refraction). Different substances have different indices of dispersion, but AFAIK they all have the same sign, and red is always refracted less than blue.
It isn’t unusual for things to have much higher index of refraction in the thermal infrared than in the visible. They also have a high index of refraction near wavelengths that they absorb strongly, which water vapor does in the thermal infrared.
I have and use a FLIR camera but have never looked for this effect. I echo the impression that poor resolution and slow frame rate would make it harder to notice. Another way to look for it might be to see if a distant straight line that crosses a close long hot object obliquely appears to bend a little. For example, if there is a slanted guy wire on a distant tower, and you look at it past a hot stove pipe, does it look kinked?
The guys at work confirmed that they get definite heat shimmer on a hot day.