From: James Rogers (jamesr@best.com)
Date: Tue Nov 30 1999 - 16:34:09 MST
I know a thing or two about gunshot imaging in IR, so I'll step into the
fray:
On Mon, 29 Nov 1999, Robin Hanson wrote:
> Hal wrote:
> >It would also be helpful in evaluating the film to know what gunfire looks
> >like on FLIR. ...
>
> I agree, but I think this may be asking too much of the Waco video producers,
> whose funding and access to FLIR equipment is presumably much less than
> the FBI and allied agencies. Hopefully a reenactment will occur.
A single gunshot, fired from a weapon with a modern flash suppressor (such
as what would be found on all rifles used at Waco), has a very distinctive
appearance when viewed with either near or far IR imaging.
A muzzle blast coming from a typical "birdcage" flash suppressor appears
as an expanding torus of light at the muzzle, oriented perpendicular to
the path of the bullet. As the torus expands, the incandescence
decreases until the fuel/air mixture drops below a level that will sustain
combustion. In the visible spectrum, a muzzle blast from this type of
suppressor generally produces a star-shaped pattern. It is interesting to
note that a flash suppressor in the visible spectrum actually increases
flash in the IR spectrum.
>From behind or in front, you see an expanding, glowing donut in the IR
spectrum. From the side, it looks like a pulse with a quick attack and
relatively slow release. The brightness, diameter, and duration of the
muzzle flash are functions of a large number of things including
ammunition used, flash suppressor design, barrel length, atmospheric
conditions, etc.
With respect to full-auto, the cyclic rate of most modern small arms is
fast enough that muzzle blasts run together between shots,
particularly if your perspective is not along the axis of the gun barrel.
Automatic fire generally appears as a rapidly pulsating light, where the
magnitude of the flash never quite reaches zero. This is primarily because
you are seeing the summed incandescence of multiple toruses, with the rate
of fade often being slower than the cyclic rate of the weapon in IR.
In the unlikely event that you have IR imaging along the axis of the gun
bore (either in front or behind) you will see concentric rings of
decreasing brightness.
> Hal wrote a bit later:
> > > technology. "Only an experiment makes sense. But the key is an experiment
> > > that takes into account the low probability of intercept: The way these
> > > cameras work, if 50 flashes were captured on film, there must have been
> > > a far higher number of gunshots that actually occurred."
> >
> >This is a surprising comment, because it seemed to me that the flashes
> >were relatively obvious and well defined compared to the background noise.
> >I'm not sure what effects he is imagining which would lead to a low
> >probability of seeing them. It almost sounds like he's doing a little
> >preparatory CYA in the event that the re-creation doesn't show flashes;
> >he can explain it as due to the "low probability of interecept".
>
> If the "shutter" were only open for a .01sec period, and then stayed closed
> until the next opening .1sec later, and if a gun flash only lasted .1 sec,
> then you should only expect to see between 10% and 20% of the flashes.
> I don't know what the actual time numbers here are, but the concern seems
> very plausible to me.
The cyclic rate of the weapons most likely used in this instance vary
from 600-900rpm.
Assuming the IR imaging was recorded at something approximating broadcast
frame rates, there should be enough samples to capture a burst of
automatic fire very nicely in most cases. But as you approach the Nyquist
limit (frame rates vs weapon cyclic rates), you will start to get aliasing
which in this case amounts to "lost" gun fire.
Individual shots fired, while probably captured (the duration of muzzle
blast should exceed reasonably fast frame rates), would have wildly
different magnitudes from shot to shot. In some cases, the muzzle blast
signature would barely register above noise floor depending on when the
frame was captured relative to when the bullet left the muzzle.
If the frame rate is relatively slow (e.g. 15 fps), you can expect to lose
a large portion of the gunshots. At 30 fps, I would expect to lose
almost none.
While some of the above characterizations may not apply in this
case (e.g. I doubt you would be able to see the torus at the distances and
resolutions involved), it should give a good baseline for discussion.
Knowing the frame rates for the IR imaging would be extremely useful in
determining if gun shots were "lost" and if lost, what percentage one
could reasonably expect to have been captured.
Regards,
-James Rogers
jamesr@best.com
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