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Chapter-14 Night Flying-V20_Sport Pilot Handbook 8/30/2021 4:09 PM Page 4

14-4 Rod Machado’s How to Fly an Airplane Handbook

Since cone cells don’t work well in Off Center Viewing at Night
the dark, it’s difficult to see an object
at night even though you’re looking
directly at it (Figure 4). If you want
the best view of a dimly lit object you
need to expose the rods to the light. Night Night
You can do this by using your periph-
eral vision for off-center viewing. Looking
Look 5 to 10 degrees to the side from offset by
the center of the object you want to view 5˚to 10˚
(Figure 5). Doing so allows some of the
Looking
object’s reflected light to fall on the rods. directly at
You can demonstrate this process at night the object
by looking directly at an airplane’s strobe
light head on, then looking at the same
light offset a few degrees. Big differ-
ence. A direct view dims the object
while an indirect view increases its
brightness.
Night Vision
How well you see at night is deter-
Fig. 5
mined by the amount of light passing
through your pupils (Figure 6). As you At night it’s much easier to see an object if you direct your vision 5°
to 10° degrees to the side of the object you’re attempting to identify.
know, pupils close to prevent the eyes from
receiving too much light and open when light intensity diminishes. They also open (dilate) to let in
more light when it’s dark. In fact, those pupils increase in diameter by a factor of five, from 2 mm
to 10 mm. This increases the light entering the eye by about 25 times. You would think that this
enormous opening would assure you adequate vision at night. It might do that but only if you haven’t
bleached a very important chemical located in those rod cells, known as rhodopsin. No, I don’t mean
The Eye’s Pupil bleaching as in someone bleaching their hair. I’m speaking of ex-
posing rhodopsin to sunlight in such a way that it prevents
your eyes from seeing properly at night regardless of their
gigantic dilated pupils.
Let’s look at the eye’s visual cycle (Figure 7) to see
how bleaching of rhodopsin occurs. When light
energy (photons) enter the eye, it stimulates the
chemical rhodopsin (position A) located in the rod
cells which are located in the retina. Rhodopsin is a
light sensitive chemical that absorbs this photonic
energy and converts it into both electrical and
chemical energy (position B).
Your pupil
The molecule cis-retinal (known to you as Vitamin
A) now begins to straighten and detach from the opsin
protein. As it does, electrical energy is released and sent
Fig. 6
© Alexandr Mitiuc - Fotolia to the brain via the optic nerve (position B). This is the
The pupil controls the light entering the eye. If electrical energy responsible for forming an image in your
your eyes have adjusted to daytime lighting, brain. The straightening cis-retinal molecule is now fully
it might take 30 minutes or more for the
eye to adapt to night conditions. straightened and is transformed into trans-retinal (position C).