[ChuckMcM]

This is pretty awesome. I mentioned doing something like this at the IoT talk I gave, I had read a paper on counting animals by their heat signature for conservation purposes and thought something along those lines would be a simple replacement for the 'walk through the door ring a bell' detectors, and given that a 32bit ARM Cortex-M is about $1.25 you could do that for not much more money than the old light + photocell.

Something the paper pointed out was that while the temperature from animal to animal varied, the same animal often kept the same temperature (+/- epsilon) when moving from sensor to sensor and that give some idea of uniqueness.

And while I also love kefka's solution I think the face recognition stuff gets a more Orwellian reaction than just tracking heat sources.