[Chaos-l] Night Vision - Self Notes

[Mark Montazer]

So, I set out to learn a bit more about night vision as I've been asked a
few times about averted vision, or why everything looks grainy at night, or
why astronomers use red flashlights. I knew the basic answers, but always
felt a bit bad giving half-answers to people who seem so interested.

I'm sure most of you already know this info, but I figured I share my
research in the off-chance it's useful or interesting, or perhaps even
needs some correcting.

Cheers,
Mark



Averted Vision

We use averted vision to observe most faint objects because the central
portion of our vision (the fovea) is densely packed with, and exclusively
occupied by, cones. The fovea is a 0.5mm diameter pit at the back of the
eye which contains about half the eye's cones (3 million of 6 million
total), absolutely no rods, and about 50% of the eye's neural connections
to the brain. It's what you use to perceive detail and, in particular to
read with. Unfortunately, cones require so much light to activate that they
are essentially useless at night. If you stare directly at faint
astronomical objects, you are physically incapable of seeing them as they
fall directly on the fovea.

Rods, on the other hand, are theoretically capable of detecting individual
photons (though 5 or 6 seem to be the realistic threshhold). The retina
outside of the fovea is far less densely packed and becomes quickly
dominated by rods the further away you get. Looking indirectly at a faint
object causes the precious photons to hit the light sensitive rods,
allowing you to better see the object.


Graininess

Though the eye contains about 120 million rods, they are spread out over a
far greater area than the rods. While cones are individually connected to
nerve fibers, dozens or even a hundred rods can be connected to a single
nerve fiber. This sparse layout, and redundant configuration, of rods is
why our night vision is grainy.


Achromatic

During the daytime we use 3 types of cones to perceive color. As there is
only one type of rod, our night vision is achromatic. This doesn't mean
it's black & white, though. In fact rods are most sensitive to blue-green
light (450-550nm) and that's why we perceive our night vision to be
slightly green, and why blue and green nebula are the easiest for us to see.


Red Sensitivity

Of the spectrum of visible light, rods are least sensitive to red. Though
this greatly hampers our ability to perceive light emissions from numerous
nebulae, it does have a unique benefit. As it happens, the fovea is packed
with 'red' cones at a ratio of 2:1 of the others. Astronomers make use of
this biological coincidence by using red light (600nm or greater) to read
by while preserving night vision.
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