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Preserving night vision

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What is the issue?
Operating a boat at night is a challenge. Typically the watchman has to keep track of the vessel's sailing environment and maintain a chart plot or at least monitor the charted position. The problem is the chart plot has to be implemented with the aid of illumination. This causes the watchman to lose their slowly acquired night vision above decks.

Why address this?
Returning above deck after a plot, and night vision will be almost completely lost. It typically takes the eyes thirty minutes to an hour to recover its biological night vision, or Scotopic Vision.

How to address this?
Use the old fashion expediency of closing one eye when coming below into the illuminated area for the plot. This should preserve most of the closed eye's night vision when it is reopened above decks.

What also helps is keeping the lights below decks at very low-light levels. A red light has been traditionally used to preserve night vision because eyes are less sensitive to this colour within longer wavelengths of light. There has been a recent shift to using green, or a blue-green light, because green allows for more visual acuity and better chart colour differentiation. Whatever colour you feel helps you more, the secret is always to use the least amount possible.

If you accidentally open both eyes another optimisation, for moving from a lighted area to dark, is to step out and close your eyes entirely counting to ten. This may sound trivial, but it does help your eyes to acclimatise. It won’t dramatically bring you to the level that thirty to sixty minutes will, but it’s better than quickly getting hit with a dark environment.

With thanks to:
Michael Harpur, Yacht Obsession.
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