Easy on the eye
How does light affect the eye of the observer, and what can be done to maintain maximum night vision? When observing the night sky, the human eye becomes nearly or completely dark-adapted, or ‘scotopic’. The scotopic eye is much more sensitive to blue and green light and much less sensitive to yellow and red light than the daytime-adapted, or ‘photopic’, eye.
Different light sources have different levels of apparent brightness to the dark-adapted eye. White light sources such as metal halide, fluorescent or white LED can produce up to three times the visual sky glow brightness of a high-pressure sodium lamp.
Astronomers tend to use a red light source to view star charts, books and notepads in the dark, as this helps to protect night vision adaption. Information printed in white text on black paper also helps maintain dark-adapted vision, because the light reflected back into the eye is considerably reduced.
What’s black and white and red all over? An astronomer’s notepad at night-time