It's not 'optical, but ...

I was told, many decades ago, by somebody who did alarm systems, that the
joint articulation of the human knew produces characteristic vigrations
which can be detected by a Doppler RADAR system as a unique signature of
human presence. Memory (decades old) says frequency is in 20-30 range
somewhere.

A simple short range Doppler RADAR can be implemented with a few cheap
transistors. Cost can be very low.




    Russell
--=20
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