Easy on the eye

Sky at Night Magazine - - DARK-SKY LIGHTING -

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

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