22
Synchronised control
Cruising Helmsman July 2018THE EYES ARE IMPORTANT, ESPECIALLY AT NIGHT AND THIS SIMPLE
ELECTRICAL CIRCUIT CHANGE COULD MAKE YOUR NIGHT VISION
THAT MUCH BETTER.
PRACTICAL
ELECTRICAL
WE have all had the experience of
walking from bright daylight into a
dark room. Initially we cannot see a thing,
then slowly the room appears to brighten
as our eyes adapt to the reduced light.
This happened to me many years ago
when I was late for the feature film and
had to find my seat in the dark. After
walking in from the mid-summer daylight
I stumbled into a row of seats and became
momentarily close to one of the seated
patrons. She was not amused and, when I
regained my dull-light vision, discovered
her male friend was equally unimpressed.This was the first time I had to really
think about my loss of vision when coming
in from the light and just how long it took
before I could clearly see.
Depending on the differences in the
relative light intensities it can actually
take up to an hour for this change to fully
take effect.
Many years later, when taking part in
cave exploration, we would sit in the dark
after entering the cave and wait several
minutes before turning on our headlights.
This helped our eyes to adapt more quickly
to the low light.of cabin and navigation lights
JOHN TYLOR
TWO EVENTS ARE OCCURRING HERE
The iris, the black bit in the middle of our
eyeball, needs to adjust to the light levels. In
bright light the iris diameter, the aperture,
reduces in size to limit the amount of light
reaching the retina, the light sensitive part
at the back of our eye that contains the
nerves to the brain. In low light it widens
to allow more light onto the retina.
The second effect involves the cells in our
eye. In our retinas we have light sensitive
cells: cones and rods, which feed information
to our brain through the optic nerve.
Cone cells work best in relatively bright
light and are responsible for our colour
vision; however they do not perform at
their best in low light. Rod cells work
well in dim light and are more sensitive in
detecting movement; they are less involved
in seeing colour. The adaptation from light
to dark can take from 30 to 60 minutes; the
transition from dark to light happens in
much less time, typically 5 minutes.