The nerve receptors in our eyes are of two types, rods and cones. Cones enable us to see colors , and are active in daylight.
Rods are for night vision , and sense shades of grey .
In the Planetarium or under the starry sky outdoors we often need to check out a star chart, use a planisphere, or read lab notes. Yet white light blinds us to the night, turning off our rods and stimulating our cones. Fortunately, red light is different, and does not impair night vision. If you have a red flashlight, you can check out your star chart and still be able to look up and see the stars. Wrap the red gel around the lens of the flashlight and secure it in place with a rubber band.