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I have a 405nm laser pointer, which emits light right on the edge of the visible spectrum. It is easy to see that this wavelength of light appears out of focus through the eye likely due to chromatic aberration. I would imagine this is because our eyes are not made to see that kind of light. I've also noticed that mercury vapor streetlights have a faint ...


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, ...

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