The perception of auras is a kind of synesthesia (emotions => vision). It can also be related to optical illusions (like contrast amplification).
Spontaneous synesthetic aura perception is very rare, and I don't know if you can develop it later on, or if secondary "aura readers" percieve the second kind, or if a strong emotion can trigger the phenomenon is someone who usually doesn't percieve it.
I've always kind of figured that the halos around holy figures had to be rooted in something real. Older cultures weren't so big on fictionalizing things. They generally drew or wrote about something real. Maybe they didn't understand it well and made up some myth to explain what it seemed like to them, the way Native Americans (of the northwest) had the idea that the world was a piece of land floating in a bowl of water and an earthquake was when one of the gods pushing down on one end of the land, causing it to dip under the water. This is a fairly accurate representation of the tsunamis that occur after coastal earthquakes. It makes sense from that perspective as an attempt to make a logical model of observed events in the face of incomplete information.
We talk about a bride "glowing" on her wedding day. Perhaps that is not merely hyperbole. Perhaps it is something subtly perceived. And perhaps halos are also something subtly perceived. Saying that makes me think of expressions like "bringer of light" or "a light upon the world".
I posted this yesterday with the title 'it turns out people glow in the dark'. Then, as now, I objected to the headline, which should have said that humans glow in the visible light spectrum. One could hardly tell this in a lit environment, could one?
Your eyes have cells which contain pigments which react upon exposure to single photons. Single photon signal is really, really noisy though and so your eye requires thousands of such signals to combine in spatially and temporally relevant patterns before it sends a signal to your brain.
Hah, cool finding. I was considering a situation more similar to "detecting one further photon of light from an already illuminated scene": just-noticeable difference instead of absolute threshold.
Back in high school we calculated the energy in a single photon by turning down the power on an LED until it almost went out. We recorded the amount of power, and figured out the energy in a photon within 15% accuracy.
