Red Flashlight

 | Basic Celestial Phenomena | Constellations | Planetarium |


Make your own red flashlight for night viewing in the planetarium or outdoors.


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. When you walk into a dark room like the Planetarium, a minimum of 20 minutes is required for one's rods fully to turn on; i.e., for your vision to become accommodated to the dark. This is why our Planetarium shows are visually rather dull and boring at first, to give everyone time to "get their night eyes."

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.


  1. Buy a small flashlight; the kind that uses two 2AA batteries works fine. No need for a searchlight; a pocket-sized light is best.
  2. Obtain from a florist or the OBU Bookstore a small piece of red gel; alternatively, eat a cinnamon candy (available from Cindy Guinn in the Science Division Office) and use the red candy wrapper.
  3. Wrap the red gel around the lens of the flashlight and secure it in place with a rubber band.
  4. Now you are ready to read star charts and lab notes without losing your night vision or blinding your neighbor! Bring your red flashlight with you whenever you come to the Planetarium or go skywatching in the outdoors.

Next: Using the Planisphere

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