Re: other infrared sources?

[Jim Houha <houha@xxxxxxxxxxx>]

Hi Neil

Those common motion detectors are called 'passive infrared' detectors. They
don't emit any light as part of the detection process (although many do
have a little LED that lights briefly to show when it has been triggered)

Inside the detector are two small thin metal plates about a quarter
centimeter apart. A simple circuit puts a small current through these two
plates and compares the resistance, which varies according to the radiation
falling on the plate. I'll let other argue whether this should properly
called a heat detector or a light detector, but basicly the light warms the
plate which varies its resistance. A cheap wide-angle faceted lens focuses
onto the plates, and is pointed so as to favor waist-level human-sized
movements. They don't want dogs and cats to trigger the detectors.

As you walk across the detectors field of view, the resistance on the
plates will vary and the detector will trigger.




>A security company gave me a demo yesterday on their system; part of it was
>dissecting one of their motion detectors.  Apparently the little snoops use
>an infrared wide angle beam to detect movement--as do other motion
>detectors.
>
>Okay, what effect, if any, will this have on pictures, loading/unloading
>film, etc.  Could this explain some of the mysterious fogging I see many of
>you writing about?
>
>Can we photograph this beam somehow?
>
>And this then begs the question: What other infrared devices abound that
>we're not aware of?
>
>Just some food for thought when y'all are out trick or treatin'.
>Happy Hallowe'en.
>
>Neil Koven
>
>
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Jim Houha                                             houha@xxxxxxxxxxx

PowerTV                                          voice     408 777-7571
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