From: Robert J. Bradbury (bradbury@www.aeiveos.com)
Date: Sat Sep 11 1999 - 21:48:13 MDT
On Sat, 11 Sep 1999, Ian Goddard wrote:
> At 02:48 PM 9/11/99 -0700, Robert J. Bradbury wrote:
> >
> >The infrared enegy coming off of an object sitting
> >in the sun can be *alot* greater than the infrared
> >energy it receives. The object absorbs the visible
> >light energy and re-emits it in the infrared.
> >Why do you think black tar or sand gets *hot*?
> >The amount of IR emitted by the object is going
> >to be proportional to its absorbance of other
> >frequencies.
> >
> > ....
>
> IAN: Reflected visible light from the sun is NOT infrared radiation,
Yes. But a mirror-type surface will relect both visible and IR radiation.
> yet you errantly think that because a body radiates IR an FLIR
> can pick up the visible light of the reflection,
No, I didn't say that. What I did say was that an FLIR can detect
the visible radiation that has been absorbed and re-emitted at a longer
wavelength (IR).
> The visible radiation reflecting off a body is not going
> to be much hotter than the IR radiaton coming off the body,
FLIR cameras are sensitive *ONLY* to infrared wavelengths.
They cannot detect *any* visible radiation reflecting of the body.
They can detect (a) visible radiation that is absorbed and
reemitted as heat; (b) heat which is generated internally
(as with human bodies).
If I have an object that is *red*, it is absorbing the blue,
green and yellow wavelengths of light. That energy doesn't
just disappear, it warms the object up and that object reemits
that radiation in the IR.
> which is why the FLIR did not pick up reflections off
> the body of water * from any angle * Hello!
The FLIR isn't going to pick up reflections off of the body
of water because the emission spectrum of a body of water
that has been heated by the sun is *dominated* by the
black body radiation curve determined by the temperature
of the water. Warm water will have a uniform "image"
to the FLIR, *unless* the power of the reflected energy
substantially exceeds the power of its natural emission
due to its temperature.
> Expert Statement:
> "...materials which reflect sunlight and thus
> seem bright in the visual spectrum will often
> appear indistinct, or even dark, to a thermal
> imager; the very reflective properties that make
> them bright to the eye make them appear cool,
> and thus dark, to FLIR systems. Interpretation
> of thermal images requires a knowledge of the
> reflective properties of both natural and
> man-made objects."
>
I agree completely with the expert opinion. Warm water
will appear bright to the FLIR. Cold water will appear
dark to the FLIR. You will only see reflections if the
power of the energy being reflected significantly exceeds
the normal emission.
Note -- I am *not* disagreeing with the assertion that any
flashes seen on the film may have been gunshots. I am simply
disagreeing with a simplified comparison of reflections
from water vs. reflections from other objects that may
have been in the pictures (windows, aluminium siding,
white paint, etc.). As the statement points out you
have to have significant knowledge of the material properties
and know what the materials were to "read" FLIR images. The
only valid comparison would be to compare normal video
images of the same locations where the FLIR detected
flashes and eliminate any possible IR reflective objects
or emitters. Then, if you could eliminate all sources
of emission or relection, you could reasonably assert
that the flashes may have been gunshots.
Robert
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