Olin Lathrop <olin_piclist <at> embedinc.com> writes:
> > It's blackbody radiation, and everything emits it and absorbs it.
> No, only truly black bodies do.  Everything else emits and absorbes less.

Less is true, but not by much. Melanine, the pigment responsible for human skin
pigmentation, is hardly an optically inactive substance, it is in fact *very*
active and its emissivity is very definitely un-blackbody-like afaik (but I do
not know in infra-red). Most complex proteins that have some *color* share this
property, among other things. Afaik that is one of the ways to look for 'life'
and plant coverage in remote sensing applications, the light emitted and
reflected by lifeforms has some serious absorbtion and emission (fluorescence)
dips and peaks in the expected blackbody plot.

> That's why air, being transparent to IR, emits virtually no IR itself.  It

Actually gases and transparent objects in general *do* emit blackbody radiation
like everything else. Transparency and emissivity are two almost unrelated
properties, as you have worked with computer graphics you should know that (in
extremes one can generate computer models of 3d ray traceable lights and halos
that are transparent yet emit quite fine). The 20K 'background radiation' of the
known sky is due mostly to this apparently. 

One of the oldest IR detectors is the Luft detector which consists of a
sensitive microphone placed in a closed chamber filled with gas (CO2 f. ex.) and
an optically chopped radiation input through a window. Works great (for the
1950s when it was all the rage and before that). The gas absorbs the radiation,
heats up and expands, then cools down and  contracts with the chopper rhythm,
creating an output on the microphone. The chamber is usually differential with
the two chambers being lit alternately and the microphone sensing differential
pressure between the chambers. The gas used selects the wavelength and the
*pressure* (thus density) is chosen as high as possible to increase sensitivity
(at the expense of the narrowness of the absorption band).

> can, however, warm or cool things it contacts and cause them to emit a
> different IR signature.  Pockets of hotter or colder air will also refract
> light, so such pockets moving in front of a IR sensor can make it look like
> the IR coming from the objects behind the pockets is changing.  This is the

Also true, but likely not the reason for the readouts from the PIR, which were
the same against the night sky as against the ceiling (in both cases detecting
breath air quite well, but under the designed pulse amplitude discriminator
threshold).

Peter


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