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Is there any facet of the energy emitted by a human body that is consistent and unique - like a fingerprint, but a signal that could be detected by a remote device?

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  • $\begingroup$ Isn't this pretty much exactly what a passive millimeter scanner does? Or ever just an infrared camera. Instead of looking for a blackbody spectrum at $T = 98.6^\circ\: \mathrm{F}$ we could just look for human-shaped outlines. $\endgroup$ Commented Dec 6, 2013 at 20:13
  • $\begingroup$ Yeah, optical wouldn't be consistent enough. And I'm curious for when optics aren't possible, like RFID $\endgroup$
    – JeremyWeir
    Commented Dec 6, 2013 at 20:25
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    $\begingroup$ Humans are reasonably close to a blackbody (barring shiny clothes). The spectral shape of the radiation is highly unlikely to be measurably different from one person to the next. This question is, however, too vague to get a good answer. $\endgroup$ Commented Dec 6, 2013 at 22:58
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    $\begingroup$ @innisfree I was wondering about recognizing Joe the plumber in a crowded room. Sort of like the idea of implanting RFID tags under people's skin. I was curious if there's something innate and unique but consistent about each human body already that could be used instead of a foreign device $\endgroup$
    – JeremyWeir
    Commented Dec 8, 2013 at 6:41
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    $\begingroup$ I think I recalled seeing something about this for facial recognition. However, this would only work in adults and would need to be updated periodically. Apparently, our faces have a unique infrared signature. Is that what you were looking for? $\endgroup$ Commented Oct 28, 2014 at 14:15

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I'm sure this isn't what you had in mind, but...

Reflected optical light (I can't think of a reason that UV wouldn't be just as good, provided there is enough UV radiation around to be reflected, e.g. sunlight) is 'radiation from a human'. A picture is a pretty good way of identifying humans. Provided the picture is sufficiently detailed (for instance, getting a good image of the iris, or a picture of some fingerprints), this is virtually guaranteed to provide a unique means of identification.

While I'm at it, I may as well quickly go over why other wavelength options you might have in mind are not as practical:

  • Radio/microwave: you could radar a human, but the resolution is roughly limited to the wavelength, so in the radio you'd be seeing at best ~1m scale features, which are roughly similar on all humans. Microwave is a bit better (mm scale), but you don't really gain anything over the optical, and lose some resolution. And I'm sure you can think of reasons that microwaving a human (at certain frequencies) might be a very bad idea.
  • Infrared: this is probably the best non-optical option. There is the advantage that humans actually emit (instead of just reflecting) at IR wavelengths, so IR imaging can be used even in the dark. The drawback is that the emission is fairly weak, so it's harder to capture detail in the IR than in a well-lit optical image.
  • X-ray: this could be a pretty good option, revealing all sorts of details that an optical image cannot, but humans don't really emit in the X-ray, which means you need a source, which makes imaging "at a distance" much more complicated, at best. There are also long-term health risks to consider.
  • $\gamma$-ray: similar problems to X-ray, plus they are more harmful.
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Indeed, the human body emits biophotons, also known as ultraweak photon emissions (UPE), with a visibility 1,000 times lower than the sensitivity of our naked eye. While not visible to us, these particles of light (or waves, depending on how you are measuring them) are part of the visible electromagnetic spectrum (380-780 nm) and are detectable via sophisticated modern instrumentation.

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  • $\begingroup$ I'm tempted to give you a +1 for teaching me something new, but I won't because upvotes are for useful answers, not just stuff I like. As stated in the comments by the OP, the question is asking for something that can "pick out Joe the plumber from a crowded room", which this answer does not address. Perhaps you might include information relating this more to the question; address whether or not biophotons are a unique signature, the ability to remotely detect them, and the limitations of using this method to identify Joe the plumber (apparently, someone wants to keep an eye on him) $\endgroup$
    – Jim
    Commented Apr 27, 2017 at 12:36
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All I can think of is EEG (electroencephalography) signal. It is a diagnostic technique that allows to monitor brain electrical activity by measuring potentials on the scalp. As I know there are some contactless methods of measuring those potentials, but I have no idea if you can measure it from big distance, since it is quite a weak signal.

This is one publication about contactless EEG: http://www.isn.ucsd.edu/pubs/embc09_eeg.pdf

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