>> A possible complication is that the cold "object" isn't a solid but
>> really a localized flux of COLD air that rapidly cross the sensor
>> area and that may be very near ( from 5 to 1 feet from the PIR).
>
>That's a problem as air is transparent to IR and doesn't exhibit any
>significant black body radiation compared to hard stuff in the room.

I heard of someone making a fire sensor that detected the hot air rising 
from a very small fire. It was designed for use in a plant, where there 
could be a small fire starting, and it was essential to detect it early and 
get it under control as the plant used a lot of combustible materials. IIRC 
it used a TV camera to sense the image changes due to the shimmer in a 
steady image from the hot air rising. Tests showed it could detect a fire 
before anyone could see the change in image.


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