The Lake County Astronomical Society

Green Light or Red?

Jack Kramer

When we need illumination during an observing session, invariably we use some sort of flashlight that produces red-colored light. Within the last few years, there has arisen a body of opinion maintaining that green light preserves your night vision as well as red. They also maintain that it provides better visibility at lower light levels. Others disagree, and the jury is still out. I use a red light at star parties because that's the norm and some might find another color intrusive. When I observe alone, I use a green light and find that it illuminates better and does not seem to have any more adverse effect on my dark adaptation than does a red light. But everyone's eyes are different, and my old eyes require more illumination to see what I'm doing. Bear in mind that light of any color adversely affects dark adaptation - different colors just have different levels of effect.

Pierre Paquette, a Canadian amateur, has done extensive research on which color of light is best. He notes: "It seems like red may indeed be better for general observation, whereas green may be better for observation of very faint objects in a very dark sky. Also, it seems like green is better for younger people, especially about faint objects in dark sky observation".

Star party etiquette dictates that any white light be prohibited and that observers use red light for illumination. Should other colors be permitted as well? (I read some time ago that the armed forces were also experimenting with blue light for night operations.) The human eye is less sensitive to the red end of the spectrum than to green light, so for now it makes sense to follow common practice in a group setting. Until someone comes up with a definitive analysis, we should probably stick with red. On your own, you might give green a try.