Definition
Sunglasses with gray-tinted lenses that reduce the overall intensity of light reaching the eye by about 85 percent without significantly altering the relative balance of colors. The N-15 designation refers to a lens that transmits approximately 15 percent of incoming light. They are the type recommended by the FAA for pilots because they protect the eyes from bright sunlight and help preserve dark adaptation needed for night flying, while keeping cockpit instruments, charts, and outside features looking their natural color.
Plain English
Gray sunglasses that cut sunlight down to about 15 percent of full strength without changing how colors look. Pilots wear them because they reduce glare and help the eyes recover for night vision while still letting you see instruments and charts in their true colors.
Context Anchor
Seen in night vision protection guidance, especially when a pilot is in bright daylight before a flight that will continue into night conditions.
Derivation
Neutral density means the lens dims all colors of light evenly — it is 'neutral' toward color and only adjusts 'density,' meaning how much light gets through. The number 15 in N-15 refers to the percent of light transmitted, so an N-15 lens lets about 15 percent of light through and blocks 85 percent.
Why Pilots Care
They prevent bright cockpit lights or ramp illumination from destroying night vision before or during flight.
Analogy
It is like turning down the brightness on a screen without changing the color settings. The picture gets darker, but the colors stay basically the same.
Intuition Check
“Neutral” does not mean the sunglasses have no effect. It means they reduce brightness evenly across colors instead of tinting the world strongly blue, yellow, or another color.
Example Sentence 1
Before her evening cross-country, she wore her N-15 neutral density sunglasses all afternoon to protect her dark adaptation for the night portion of the flight.
Example Sentence 2
During the night preflight, she kept her N-15 sunglasses on while checking the fuel to avoid losing dark adaptation.