On Sat, 11 Sep 1999 22:05:21 -0400 Ian Goddard <Ian@Goddard.net> writes:
> At 02:48 PM 9/11/99 -0700, Robert J. Bradbury wrote:
>
> >I think you need to study some physics about
> >light, heat and reflections.
> >
> >The infrared energy 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, yet you errantly
> think that because a body radiates IR an FLIR
> can pick up the visible light of the reflection,
> it does not. The visible radiation reflecting
> off a body is not going to be much hotter than
> the IR radiation coming off the body, which is
> why the FLIR did not pick up reflections off
> the body of water * from any angle * Hello!
>
>
> >I would leave the determination of reflections vs.
> >gun muzzle flashes to people who are trained to
> >interpret FLIR images.
>
> IAN: Hubba hubba. Fine, then listen to Doctor
> Edward Allard, who not only worked much of his
> life for the military in IR imaging but also
> holds patents in IR-imaging technology:
>
> "...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."
>
> http://www.indirect.com/www/dhardy/allard.html
>
'Infrared' and 'visible' are just names we give to certain rather arbitrarily defined parts of the electromagnetic spectrum. For example, the visible spectrum is rather narrow, less than an octave, and is based on the photoptic spectral response of the eye. That response starts at zero response at about 700 nm (where red drops to black), peaks at about 555 or 560 nm (green) and goes back to zero response at about 400 nm (where violet drops to black).
To interpret FLIR pictures, we need to have data on the solar spectrum at ground level as a function of sun altitude and atmospheric conditions, the spectral response of the FLIR instrumentation, the spectral reflectance and absorbance of the materials involved, whether and to what extent the reflectance is specular or diffuse, the temperatures and consequent IR signatures of the materials, specific angles of incidence and aspect for important surfaces... the list goes on and on.
So if an object gives off lots of IR, it would not necessarily appear correspondingly bright in FLIR pictures unless it gives off lots of IR within the band of sensitivity of the FLIR camera. And an object which emits only a little IR could appear disproportionately bright on FLIR if the wavelengths of IR it emits correspond closely with the peak sensitivity of the FLIR camera.
Experimental data (such as what a gun muzzle flash and a specular reflectance flash look like on FLIR) and experience in interpreting FLIR pictures are also important. So while anyone may look at the FLIR film and draw their own conclusions, in the end the interpretations of experts will carry more weight, though we must remember that an expert is not immune from bias.
Ron Kean
.
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